updating user verbs documentation

[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. 2nd
patch links this file to the main index so we can actually find
it by browsing (search will work in any case). 3rd 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.

[PATCH 1/3] docs-rst: infiniband: Convert user verbs doc to rst
[PATCH 2/3] docs-rst: update index file with infiniband docs
[PATCH 3/3] docs-rst: infiniband: update verbs API details