Help: Improve documentation of project SPDX_LICENSE

Currently, CMake allows specifying a license (`SPDX_LICENSE`) for both
projects and targets, which is largely motivated by CPS export. The
design of CPS is geared toward components (targets) only expressing an
explicit license when that component's license differs from the license
that applies to the package as a whole (or, at least, the license
generally used by the package's software components). Accordingly, the
project license is not used to initialize the `SPDX_LICENSE` target
property at export time. This, however, was not documented previously.

Add a note explaining this to the `SPDX_LICENSE` option of the `project`
command.

Fixes: #27295