Patch: Make pg_waldump.sgml less confusing

The pg_waldump manpage says the --save-fullpage output format was

TIMELINE-LSN.RELTABLESPACE.DATOID.RELNODE.BLKNOFORK

when the filenames actually have the format

00000001-00000000-309A88C0.1663.5.64080.0_main

with an underscore in front of FORK.

While that is technically correct because it then explains that
FORK is something like "_main", I think this is just confusing and
moving the underscore into the file name format description is much
cleaner. (And inside the rest of the code, the fork names don't have
underscore prefixes either.)

Christoph