MINOR: doc: elaborate on diagnostic mode behavior

MINOR: doc: elaborate on diagnostic mode behavior

Commit 7b01a8dbdd("MINOR: global: define diagnostic mode of execution")
in 2.4-dev15 introduced diagnostic mode for config warnings.

It was initially designed to be ignored by "zero-warning" mode
(-dW) and documented accordingly.

Since commit ced4148401 ("BUG/MINOR: diag: always show the version
before dumping a diag warning") in 3.0-dev3 this changed, and
zero-warning mode combined with diagnostic messages stop haproxy from
starting. This appears to be a unintended consequence in ced4148401.

Since commit 7d63efa5f5 ("MINOR: errors: add ha_diag_notice() to
report diag-level notifications") there is a distinction between
what should not interact with zero-warning mode (ha_diag_notice()),
and the previous behavior with ha_diag_warning().

Explain the that some diagnostic message interact with zero-warning
and that diagnostic message will be backported, as such diagnostic
mode must only be used interactively.

Should be backported to stable trees.

Co-Authored-By: Willy Tarreau <w@1wt.eu>