Docs - fix up sample source/filter tutorial

Author: samaaron

etc/doc/tutorial/en/03.7-Sample-Packs.md

@@ -93,9 +93,9 @@ names in our directory:
 Notice that in these filenames we have quite a bit of
 information. Firstly, we have the BPM of the sample (beats per minute)
 at the start. So, the piano sample is at 120 BPM and our first three
-melodies are at 100 BPM. Also, our sample names contain the key. So
-the guitar sample is in Bb and the melodies are in A#. This information
-is very useful for mixing in these samples with our other code. For
+melodies are at 100 BPM. Also, our sample names contain the key. So the
+guitar sample is in Bb and the melodies are in A#. This information is
+very useful for mixing in these samples with our other code. For
 example, we know we can only play the piano sample with code that's in
 120 BPM and in the key of Bb.
 
@@ -177,6 +177,9 @@ Take care though as searching through a very large set of folders may
 take a long time. However, the contents of all folder sources are
 cached, so the delay will only happen the first time.
 
+Finally, note that the sources *must go first*. If no source is given,
+then the set of built-in samples will be selected as the default list of
+candidates to work with:
 
 ## Filters
 
@@ -186,16 +189,26 @@ types to further reduce the selection:
 * `"foo"` Strings will filter on substring occurrence within file name (minus directory path and extension).
 * `/fo[oO]/` Regular Expressions will filter on pattern matching of file name (minus directory path and extension).
 * `:foo` - Keywords will filter candidates on whether the keyword is a direct match of the filename (minus directory path and extension).
-* `lambda{ "foo" }` - Procs with no arguments will be automatically called and their result will be treated as a source or filter.
 * `lambda{|a| ... }` - Procs with one argument will be treated as a candidate filter or generator function. It will be passed the list of current candidates and must return a new list of candidates (a list of valid paths to sample files).
 * `1` - Numbers will select the candidate with that index (wrapping round like a ring if necessary).
 
+For example, we can filter over all the samples in a directory
+containing the string \"foo\" and play the first matching sample at half
+speed:
+
+```
+sample \"/path/to/samples\", \"foo\", rate: 0.5
+```
+
+See the help for `sample` for many detailed usage examples. Note that
+the ordering of the filters is honoured.
+
 ## Composites
 
 Finally, you may use lists wherever you may place a source or
-filter. The list will be automatically flattened and the contents will be
-treated as regular sources and filters. Therefore the following calls to
-`sample` are semantically equivalent:
+filter. The list will be automatically flattened and the contents will
+be treated as regular sources and filters. Therefore the following calls
+to `sample` are semantically equivalent:
 
 ```
 sample "/path/to/dir", "100", "C#"