pgsql: Improve file header comments for astramer code.

Improve file header comments for astramer code.

Make it clear that "astreamer" stands for "archive streamer".
Generalize comments that still believe this code can only be used
by pg_basebackup. Add some comments explaining the asymmetry
between the gzip, lz4, and zstd astreamers, in the hopes of making
life easier for anyone who hacks on this code in the future.

Robert Haas, reviewed by Amul Sul.

Discussion: http://postgr.es/m/CAAJ_b97O2kkKVTWxt8MxDN1o-cDfbgokqtiN2yqFf48=gXpcxQ@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/22b4a1b561f830f2af80c21450a4c05f6efbb698

Modified Files
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src/fe_utils/astreamer_file.c    |  4 ++++
src/fe_utils/astreamer_gzip.c    | 15 +++++++++++++++
src/fe_utils/astreamer_lz4.c     |  4 ++++
src/fe_utils/astreamer_zstd.c    |  4 ++++
src/include/fe_utils/astreamer.h | 21 +++++++++++++++------
5 files changed, 42 insertions(+), 6 deletions(-)