further document shm code

[i509VCB]

Fixes #680 Fixes an issue where the fd is leaked if creating a pool breaks.

This pull request documents the internals of the pool module inside the shm module of the wayland frontend.

This also changes the internal memory mapping of the pool to be guarded by a Mutex to allow future pull requests to expose writing to a pool (required for wlr_screencopy).

This also cleans up the code to prepare to write a draft implementation for https://gitlab.freedesktop.org/wayland/wayland/-/merge_requests/161