Extend the documentation added by commit 96a7040107 (project: Define
variables indicating whether project is top level, 2021-03-24,
v3.21.0-rc1~443^2) to give some examples of how the variables are set in
each context.
Define `PROJECT_IS_TOP_LEVEL` and `<PROJECT-NAME>_IS_TOP_LEVEL`. The
latter is a STATIC cache entry just like other `<PROJECT-NAME>_*`
variables so that it is globally scoped.
Issue: #20310Fixes: #21961