docs-rst: update infiniband uverbs doc

[Joel Nider]

A small patchset to update the verbs API documentation with some
information regarding the ioctl syscall. First patch converts the
file format to ReST, since this is the new preferred format, moves
the file to Documentation/userspace-api, and updates the index.
The 2nd patch adds the new content, documenting a bit of the internal
workings of the kernel side of the API functions. The goal is to make
it easier for developers unfamiliar with the structure to understand
what is going on when adding a new function.