I chose the most common one here, although the indentation is a bit odd.

The previous style(s) used in most places here violates the C standard, and in
practice can trigger -Wreserved-id-macro in clang for client code.  So, switch
to a conventional style that doesn't trigger warnings (and is supported by
tools like clang-tidy).

Collectively, these trigger warnings in both Doxygen and
clang (-Wdocumentation-unknown-command).  Weirdly, @p in doxygen doesn't do
anything but print in a fixed-width font anyway, so just use Markdown quotes to
replace it which has the same effect (and reads nicer in the source).  The use
of @param like @p was invalid, so do the same to those.

This fixes a Doxygen warning where a (seemingly) non-existent parameter was
documented.

Including C headers inside of `extern "C"` breaks use from C++. Hoist
the includes of standard C headers above the block so we don't try
to mangle the stdlib.

I initially tried to scope this with a targeted change but it's too
hard to do correctly that way. This way, we avoid whack-a-mole.

Firefox is working around this in their e21461b7b8b39cc31ba53c47d4f6f310c673ff2f
commit.

Bug: https://bugzilla.mozilla.org/1953080