ext4: add missing bigalloc documentation.

There was a broken link for bigalloc.  The page
https://ext4.wiki.kernel.org/index.php/Bigalloc

 was not migrated into
the current documentation sources.  This patch adds the contents of
that missing page into the section for Bigalloc itself.

Signed-off-by: Ayush Ranjan <ayushr2@illinois.edu>
Link: https://lore.kernel.org/r/20190831154419.GA30357@fa19-cs241-404.cs.illinois.edu


Signed-off-by: Theodore Ts'o <tytso@mit.edu>