Copyedit the proof mode documentation

See merge request FP/iris-coq!200

This became broken after the nested iSpecialize MR.

Fix issue #206

Closes #206

See merge request FP/iris-coq!199

Allow `iSpecialize` to be nested.

See merge request FP/iris-coq!198

It now supports the specialization pattern `(H spat1 .. spatn)`, which first
recursively specializes the hypothesis `H` using the specialization patterns
`spat1 .. spatn`.

This commit closes issue #192.

The continuation is called with a Boolean indicating whether the
hypothesis was in the intuitionistic context or not.

Split it up into more logical parts.

The unbounded fractional camera

See merge request FP/iris-coq!195

- More consistent indentation.
- Mark new subgoals as comments.

Lemmas about subst_map on closed expressions

See merge request FP/iris-coq!194

Two small lemmas about binder_insert

See merge request FP/iris-coq!193