Improve comment about snapshot macros

Improve comment about snapshot macros

The comment mistakenly had "the others" for "the other", but this
commit also reorders the comment so it matches the macros below.  Now we
describe the levels in increasing strictness.  In addition, it seems
easier to follow if we introduce one level at a time, rather than
describing two, followed by "the other" (and then jumping back to one of
the first two).  Finally, reword the sentence about the purpose of the
macros, which was slightly off-point.

Author: Paul Jungwirth <pj@illuminatedcomputing.com>
Reviewed-by: Rustam ALLAKOV <rustamallakov@gmail.com>
Reviewed-by: Dean Rasheed <dean.a.rasheed@gmail.com>
Reviewed-by: Robert Haas <robertmhaas@gmail.com>
Discussion: https://postgr.es/m/CA+renyUp=xja80rBaB6NpY3RRdi750y046x28bo_xg29zKY72Q@mail.gmail.com