Add some text to expression evaluator documentation

[tromey]

No description provided.

[coveralls]

Coverage remained the same at 93.791% when pulling eec7aa1 on tromey:eval-doc-additions into e3ab3cf on gimli-rs:master.

[jonhoo]

While we're updating this, I'd like to also see a somewhat longer description for set_max_iterations. In particular, it is unclear what exactly "iterations" means. Also, the use of None in the description doesn't match the interface (which takes a u32, not an Option<u32>).

[tromey]

Thanks for the suggestions! I'll fix up this patch tomorrow.

[tromey]

For the None thing, what is going on is that the API is asymmetric -- you can set the max iterations but you can't clear it. This capability didn't seem very important so I left it out.

[jonhoo]

@tromey I think that's fine (not letting the user clear it), but I think it is then not right for the method documentation to talk about None.

[coveralls]

Coverage remained the same at 93.791% when pulling 98c1aa5 on tromey:eval-doc-additions into e3ab3cf on gimli-rs:master.

[jonhoo]

LGTM, r+

[fitzgen]

Thanks! :)