v2: docs expansion + source comments

[robbielyman]

currently i'm pretty happy with the state of comments in a couple of the core Zig files (in the v2 branch). would be good to maintain approximately that level of chattiness throughout the project.

pretty happy with Ldoc currently, but seamstress v2 could use more longform documentation on par with @dndrks's excellent study but maybe going slightly deeper? wider? hard to say. could also enforce an Ldoc build finishing without errors as a CI step...?