Make the README and docs intro more "ARG"y (#3332)

Moving to a phrasing where the tree sequence is a format for storing genetic genealogies / ARGs.

Now that we have the ARG interface (for going up and down the ARG), perhaps we don't want to say that the encoding is focussed on storing the correlated trees?

Author: hyanwong

README.md

@@ -11,22 +11,21 @@
 [![Docs Build](https://github.com/tskit-dev/tskit/actions/workflows/docs.yml/badge.svg)](https://github.com/tskit-dev/tskit/actions/workflows/docs.yml) [![Binary wheels](https://github.com/tskit-dev/tskit/actions/workflows/wheels.yml/badge.svg)](https://github.com/tskit-dev/tskit/actions/workflows/wheels.yml) [![Tests](https://github.com/tskit-dev/tskit/actions/workflows/tests.yml/badge.svg)](https://github.com/tskit-dev/tskit/actions/workflows/tests.yml)
 
 
-Succinct tree sequences are a highly efficient way of storing a set of related DNA
-sequences by encoding their ancestral history as a set of correlated trees along the
-genome. The tree sequence format is output by a number of software libraries and programs
-(such as [msprime](https://github.com/tskit-dev/msprime),
+The succinct tree sequence (`tskit) format is an efficient way of representing
+the genetic history - sometimes known as an Ancestral Recombination Graph or ARG -
+of a set of related DNA sequences. `Tskit` is used
+by a number of software libraries and programs (such as
+[msprime](https://github.com/tskit-dev/msprime),
 [SLiM](https://github.com/MesserLab/SLiM),
 [fwdpp](http://molpopgen.github.io/fwdpp/), and
 [tsinfer](https://tskit.dev/tsinfer/docs/stable/)) that either simulate or infer
-the evolutionary history of genetic sequences. The evolutionary history of genetic
-sequences is often technically referred to as an Ancestral Recombination Graph (ARG);
-succinct tree sequences are fully compatible with this formulation, and tskit is a
-therefore a powerful platform for processing ARGs.
+the evolutionary ancestry of genetic sequences.
 
 The `tskit` library provides the underlying functionality used to load, examine, and
-manipulate tree sequences, including efficient methods for calculating genetic
-statistics. It often forms part of an installation of other software packages such as
-those listed above. Please see the
+manipulate ARGs in the tree sequence format, including efficient access to the
+sequence of correlated trees along a genome and general methods to calculate
+genetic statistics. `Tskit` often forms part of an installation of other
+software packages such as those listed above. Please see the
 [documentation](https://tskit.dev/tskit/docs/stable/) for further details, which
 includes
 [installation instructions](https://tskit.dev/tskit/docs/stable/installation.html).

docs/introduction.md

@@ -18,26 +18,25 @@ kernelspec:
 
 # Introduction
 
-This is the documentation for `tskit`, the tree sequence toolkit. Succinct tree sequences
-provide a highly efficient way of storing a set of related DNA sequences by encoding
-their ancestral history as a set of correlated trees along the genome.  The evolutionary
-history of genetic sequences is often technically referred to as an Ancestral
-Recombination Graph (ARG); succinct tree sequences are fully compatible with this
-formulation, and tskit is a therefore a powerful platform for processing ARGs.
+This is the documentation for `tskit`, the tree sequence toolkit.
+Succinct tree sequences are an efficient way of representing the
+genetic history - often technically referred to as an Ancestral
+Recombination Graph or ARG - of a set of DNA sequences.
 
 The tree sequence format is output by a number of external software libraries
 and programs (such as [msprime](https://tskit.dev/msprime/docs), 
 [SLiM](https://github.com/MesserLab/SLiM), 
 [fwdpp](https://fwdpp.readthedocs.io/en/), and 
 [tsinfer](https://tskit.dev/tsinfer/docs/)) that either simulate or
-infer the evolutionary history of genetic sequences. This library provides the
+infer the evolutionary ancestry of genetic sequences. This library provides the
 underlying functionality that such software uses to load, examine, and
-manipulate tree sequences, including efficient methods for calculating
+manipulate ARGs in tree sequence format, including efficient access to the
+correlated sequence of trees along a genome and general methods to calculate
 {ref}`genetic statistics<sec_stats>`.
 
 For a gentle introduction, you might like to read "{ref}`tutorials:sec_what_is`"
 on our {ref}`tutorials site<tutorials:sec_intro>`. There you can also find further
-tutorial material to introduce you to the key concepts behind succinct tree sequences.
+tutorial material to introduce you to key `tskit` concepts.
 
 :::{important}
 If you use `tskit` in your work, please remember to cite it appropriately: see the {ref}`citations<sec_citation>` page for details.