docs: Remove XML-style comments from documentation strings

gtk-doc doesn’t support them any more since it was ported to Markdown,
so they end up appearing in the generated documentation, which isn’t
great.

Mostly, they were used to split up things invisibly, which we can do in
other ways.

Signed-off-by: Philip Withnall <withnall@endlessm.com>
Reviewed-by: nobody

glib/ghash.c

@@ -118,7 +118,7 @@
  * g_direct_hash() is also the appropriate hash function for keys
  * of the form `GINT_TO_POINTER (n)` (or similar macros).
  *
- * <!-- FIXME: Need more here. --> A good hash functions should produce
+ * A good hash functions should produce
  * hash values that are evenly distributed over a fairly large range.
  * The modulus is taken with the hash table size (a prime number) to
  * find the 'bucket' to place each key into. The function should also