Document mutation mode and change site mode description

[gregorgorjanc]

Mutation mode is described in #3299 and related work is in #3266 and #3271.

The current docs say site: How many mutations differentiate these two genomes?

tskit/docs/stats.md

Line 212 in e4ca469

: How many mutations differentiate these two genomes?

. This should be something like `site: How many alleles differentiate these two genomes?.

Similarly at

tskit/docs/stats.md

Line 224 in e4ca469

(`site`) count the number of mutations falling on the paths,

Best to search for all site hits in the stats.md file (possibly elsewhere).

[jeromekelleher]

Might also be useful to have a sentence somewhere saying "This is perhaps best thought of as 'state mode', where only the allelic state of samples is considered, not the underlying genealogies", and sort of implying that if we were doing this again we'd pick a different name for it.

[gregorgorjanc]

Probably also add the nice scheme figure from @petrelharp at #3299 (comment) - that really nicely shows the difference between the nodes.