Document Dynamic BuildRequires feature

[praiskup]

This is a pretty complicated feature, and we should document peculiarities, like the one in https://rpm-software-management.github.io/mock/Release-Notes-5.3

The other thing worth documenting is that the output from %generate_buildrequires needs to provide stable output eventually, after doing the N installation iteration steps, no matter N value (may be zero for a cached buildroot).

The #1352 tries to achieve a good goal; WRT %generate_buildrequires -> it seems we could provide guidance that it is not a good idea to modify %prep output (source directory) by %generate_buildrequires. Or expect that such changes are not reverted before the %build starts.

[praiskup]

While reporting rpm-software-management/rpm#3120, I noticed there's a good RPM buildprocess documentation now. We should mention it somewhere.

[praiskup]

See also discussion in: rpm-software-management/rpm#3091