pgsql: doc: Clarify how to generate backup files with non-exclusive bac

doc: Clarify how to generate backup files with non-exclusive backups

The current instructions describing how to write the backup_label and
tablespace_map files are confusing. For example, opening a file in text
mode on Windows and copy-pasting the file's contents would result in a
failure at recovery because of the extra CRLF characters generated. The
documentation was not stating that clearly, and per discussion this is
not considered as a supported scenario.

This commit extends a bit the documentation to mention that it may be
required to open the file in binary mode before writing its data.

Reported-by: Wang Shenhao
Author: David Steele
Reviewed-by: Andrew Dunstan, Magnus Hagander
Discussion: https://postgr.es/m/8373f61426074f2cb6be92e02f838389@G08CNEXMBPEKD06.g08.fujitsu.local
Backpatch-through: 9.6

Branch
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REL_12_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/677357047b9c920e351b73d9670a813fcdc6801b

Modified Files
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doc/src/sgml/backup.sgml | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)