BreakingChanges: clarify the procedure

The point behind a compile-time switch is to ensure that we have a
mechanism to hide myriad of backward incompatible changes that may
be prepared and accumulated over time, yet make them available for
testing any time during the development toward the big version
boundary.  Add a few words to stress that point.

Since the document was first written, we have added the CI job that
the document anticipated us to have.  Rephrase to state the current
status.

The discussion in [*1*] made us abandon the "feature.git3" based
runtime switching of behaviour and instead adopt the compile-time
switching mechanism, but a stray sentence about runtime switching
still remained in the final text by mistake.  Remove it.

[Reference]

 *1* https://lore.kernel.org/git/xmqqldzel6ug.fsf@gitster.g/

Signed-off-by: Junio C Hamano <gitster@pobox.com>
Acked-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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