docs: Hide Unix and Win32 symbols from main GLib and GIO docs

These symbols are documented in the {glib,gio}-{unix,win32}
documentation builds, and shouldn’t be duplicated in the main
documentation. It’s a historical (and unavoidable) accident that those
symbols are in the main GIR files.

If we don’t hide the symbols from the main docs build, we now run into
the problem of the `GioUnix` namespace not being known to gi-docgen
while building the `Gio` docs. This is because of the previous two
commits, which ported more of the `gunixmounts` documentation to
gi-docgen syntax. I don’t know of another way to fix this, other than to
hide the `GioUnix` symbols from the `Gio` docs build.

Signed-off-by: Philip Withnall <[email protected]>

Helps: #3250