Update usage.md

Author: pinin4fjords

docs/usage.md

@@ -128,11 +128,11 @@ The `--aligner hisat2` option is not currently supported using ARM architecture
 
 By default, the pipeline uses [STAR](https://github.com/alexdobin/STAR) (i.e. `--aligner star_salmon`) to map the raw FastQ reads to the reference genome, project the alignments onto the transcriptome and to perform the downstream BAM-level quantification with [Salmon](https://salmon.readthedocs.io/en/latest/salmon.html). STAR is fast but requires a lot of memory to run, typically around 38GB for the Human GRCh37 reference genome. Since the [RSEM](https://github.com/deweylab/RSEM) (i.e. `--aligner star_rsem`) workflow in the pipeline also uses STAR you should use the [HISAT2](https://ccb.jhu.edu/software/hisat2/index.shtml) aligner (i.e. `--aligner hisat2`) if you have memory limitations.
 
-You also have the option to pseudoalign and quantify your data directly with [Salmon](https://salmon.readthedocs.io/en/latest/salmon.html) or [Kallisto](https://pachterlab.github.io/kallisto/) by specifying `salmon` or `kallisto` to the `--pseudo_aligner` parameter. The selected pseudoaligner will then be run in addition to the standard alignment workflow defined by `--aligner`, mainly because it allows you to obtain QC metrics with respect to the genomic alignments. However, you can provide the `--skip_alignment` parameter if you would like to run Salmon or Kallisto in isolation. By default, the pipeline will use the genome fasta and gtf file to generate the transcripts fasta file, and then to build the Salmon index. You can override these parameters using the `--transcript_fasta` and `--salmon_index` parameters, respectively. By default, when specifying `--pseudo_aligner salmon` without an index, even with `--skip_alignment set` Salmon will still use the genomic FASTA file when building an index, providing the sequences as 'decoys' (see [Salmon documentation](https://salmon.readthedocs.io/en/latest/salmon.html#preparing-transcriptome-indices-mapping-based-mode)), and this is the recommended mode of operation in this situation. However, if you do not supply a FASTA file, Salmon will run without those decoys, using only transcript sequences in the index.
+You also have the option to pseudoalign and quantify your data directly with [Salmon](https://salmon.readthedocs.io/en/latest/salmon.html) or [Kallisto](https://pachterlab.github.io/kallisto/) by specifying `salmon` or `kallisto` to the `--pseudo_aligner` parameter. The selected pseudoaligner will then be run in addition to the standard alignment workflow defined by `--aligner`, mainly because it allows you to obtain QC metrics with respect to the genomic alignments. However, you can provide the `--skip_alignment` parameter if you would like to run Salmon or Kallisto in isolation. By default, the pipeline will use the genome fasta and gtf file to generate the transcripts fasta file, and then to build the Salmon index. You can override these parameters using the `--transcript_fasta` and `--salmon_index` parameters, respectively. 
 
 The library preparation protocol (library type) used by Salmon quantification is inferred by the pipeline based on the information provided in the samplesheet, however, you can override it using the `--salmon_quant_libtype` parameter. You can find the available options in the [Salmon documentation](https://salmon.readthedocs.io/en/latest/library_type.html). Similarly, strandedness is taken from the sample sheet or calculated automatically, and passed to Kallisto on a per-library basis, but you can apply a global override by setting the Kallisto strandedness parameters in `--extra_kallisto_quant_args` like `--extra_kallisto_quant_args '--fr-stranded'` see the [Kallisto documentation](https://pachterlab.github.io/kallisto/manual).
 
-When running Salmon in mapping-based mode via `--pseudo_aligner salmon` the entire genome of the organism is used by default for the decoy-aware transcriptome when creating the indices (see second bulleted option in [Salmon documentation](https://salmon.readthedocs.io/en/latest/salmon.html#preparing-transcriptome-indices-mapping-based-mode)).
+When running Salmon in mapping-based mode via `--pseudo_aligner salmon`, supplying a genome fasta via `--fasta` and not supplying a Salmon index, the entire genome of the organism is used by default for the decoy-aware transcriptome when creating the indices, as is recommended (see second bulleted option in [Salmon documentation](https://salmon.readthedocs.io/en/latest/salmon.html#preparing-transcriptome-indices-mapping-based-mode)). If you do not supply a FASTA file or an index, Salmon will index without those decoys, using only transcript sequences in the index. This second option is not usually recommended, but may be useful in limited circumstances. Note that Kallisto does not index with genomic sequences.
 
 Two additional parameters `--extra_star_align_args` and `--extra_salmon_quant_args` were added in v3.10 of the pipeline that allow you to append any custom parameters to the STAR align and Salmon quant commands, respectively. Note, the `--seqBias` and `--gcBias` are not provided to Salmon quant by default so you can provide these via `--extra_salmon_quant_args '--seqBias --gcBias'` if required. You can now also supply additional arguments to Kallisto via `--extra_kallisto_quant_args`.
 
@@ -346,7 +346,7 @@ nextflow run \
     -profile docker
 ```
 
-This is not usually recommended with Salmon unless you also supply a previously generated decoy-aware Salmon transcriptome.
+This is not usually recommended with Salmon unless you also supply a previously generated decoy-aware Salmon transcriptome index.
 
 > **NB:** Loading iGenomes configuration remains the default for reasons of consistency with other workflows, but should be disabled when not using iGenomes, applying the recommended usage above.