pgsql: Add macro for customizing an archiving WARNING message.

Add macro for customizing an archiving WARNING message.

Presently, if an archive module's check_configured_cb callback
returns false, a generic WARNING message is emitted, which
unfortunately provides no actionable details about the reason why
the module is not configured.  This commit introduces a macro that
archive module authors can use to add a DETAIL line to this WARNING
message.

Co-authored-by: Tung Nguyen
Reviewed-by: Daniel Gustafsson, Álvaro Herrera
Discussion: https://postgr.es/m/4109578306242a7cd5661171647e11b2%40oss.nttdata.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/2c29e7fc95b24f5ccfec0d2db458d2130606f446

Modified Files
--------------
contrib/basic_archive/basic_archive.c |  7 ++++++-
doc/src/sgml/archive-modules.sgml     | 12 ++++++++++++
src/backend/archive/shell_archive.c   |  7 ++++++-
src/backend/postmaster/pgarch.c       |  8 +++++++-
src/include/archive/archive_module.h  |  8 ++++++++
5 files changed, 39 insertions(+), 3 deletions(-)