clarify that docs must be built before make test

[brettsheffield]

See comment in #16650

Running make test prior to running make doc results in this error:

added 3 packages and updated 1 package in 1.717s
/bin/sh: out/doc/api/console.html: No such file or directory
Makefile:579: recipe for target 'out/doc/api/console.html' failed
make[1]: *** [out/doc/api/console.html] Error 1
Makefile:213: recipe for target 'test' failed
make: *** [test] Error 2

After running make doc, make test succeeds.

This PR updates BUILDING.md to clarify.

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• documentation is changed or added
• commit message follows commit guidelines

Affected core subsystem(s)

[joyeecheung]

Sorry, this is not the intended behavior and should be fixed by #16661 ..

[brettsheffield]

Ok. Thanks for clarifying @joyeecheung. If the intention is to fix the behavior, I'll close this pull request.