[BUGFIX] alt_doc.php uses deleted alternative page languages for translations

The backend script alt_doc.php even considers deleted
pages_language_overlay records when determining in which language a
record can be/is localized.

This patch improves method documentation and applies
t3lib_BEfunc::deleteClause() on the respective database query.

Change-Id: I2f34cfb888a452136120b9e5d94cf2866b909fb1
Fixes: #31379
Releases: 4.3, 4.4, 4.5, 4.6, 4.7
Reviewed-on: http://review.typo3.org/6386
Reviewed-by: Helmut Hummel
Tested-by: Helmut Hummel
Reviewed-by: Markus Klein
Tested-by: Markus Klein
Reviewed-by: Steffen Ritter
Tested-by: Steffen Ritter