[Rpm-maint] [rpm-software-management/rpm] The behavior of `%{?builtin_macro:foo}` is unclear from documentation (Issue #1866)

[Vít Ondruch]

Based on #1864, I'd like the documentation to be clarified what does the `?` do with builtin macros. The documentation currently says:

> both macro “%{?builtin_macro:value}” and its negative version “%{?!builtin_macro:value}” are expanded exactly like the macro without exclamation mark and question mark “%{builtin_macro:value}”

Given how the other macros work, I'd expect that adding the `?` would just soften the behavior, so there would not be failure if the built in macro is not available. Or IOW, the behavior of `%{?builtin_macro:value} == %{builtin_macro:value}`. But it does something different. So lets try it on some random spec and add there the following line:

~~~
%{exists:%{SOURCE4}}
~~~

The `fedpkg srpm` fails with following error:

~~~
error: line 112: Unknown tag: 1
~~~

Ok, so the macro expands to `1`, so the `SOURCE4` file exists. So far so good.

Now lets try the question mark:

~~~
error: line 112: Unknown tag: /home/vondruch/fedora-scm/own/ruby/macros.ruby
~~~

Hm, so it expands the `SOURCE4` now, but it does not tell anything about functionality. So is the following the correct way to ensure the `exists` does not fail even when not available?

~~~
%{?exists:%{exists:%{SOURCE4}}}
~~~

This results in `error: line 112: Unknown tag: 1` so pretty similar to the initial version.

Nevertheless, I in any way, am not able to decipher this behavior from the initial description. In my opinion, the original descriptions should be modified to following:

~~~
The macro “%{?builtin_macro:value}” is expanded into "value" when the `builtin_macro` is available/implemented.
~~~

And I am quite unsure what is the result of `%{?!builtin_macro:value}`. For available builtins, this is probably no-op.

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