pgsql: Rephrase references to "time qualification".

Rephrase references to "time qualification".

Now that the relevant code has, for other reasons, moved out of
tqual.[ch], it seems time to refer to visiblity rather than time
qualification.

Author: Andres Freund
Discussion: https://postgr.es/m/20180703070645.wchpu5muyto5n647@alap3.anarazel.de

Branch
------
master

Details
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https://git.postgresql.org/pg/commitdiff/ebcc7bf949bae614cccc6b86e3ef9b926a6e2f99

Modified Files
--------------
doc/src/sgml/indexam.sgml                   | 2 +-
src/backend/access/heap/heapam.c            | 6 +++---
src/backend/access/heap/heapam_visibility.c | 4 ++--
src/backend/utils/cache/inval.c             | 2 +-
4 files changed, 7 insertions(+), 7 deletions(-)