pgsql: doc: Add acronyms for MITM and SNI

doc: Add acronyms for MITM and SNI

This adds MITM and SNI as acronyms, as the documentation already had
them marked up with <acronym>.

While on it, make sure to spell man-in-the-middle with dashes
consistently, and add acronyms for those new terms where appropriate.

Author: Daniel Gustafsson
Reviewed-by: Michael Paquier
Discussion: https://postgr.es/m/CE12DD5C-4BB3-4166-BC9A-39779568734C@yesql.se

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/15ff5401d1719aaf6c9a47e5abea517cc2bcbaf1

Modified Files
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doc/src/sgml/acronyms.sgml | 23 +++++++++++++++++++++++
doc/src/sgml/config.sgml   |  2 +-
doc/src/sgml/libpq.sgml    | 13 ++++++-------
3 files changed, 30 insertions(+), 8 deletions(-)