Help: Fix wrong documented return values for unset inherited properties

Since !1879 (merged), the docs for some get_..._property() commands incorrectly describe the behavior for inherited properties. When a property is not set, even in a parent scope, the returned result from the get_..._property() command is the same whether the property is inherited or not. The docs incorrectly stated that an empty string would be returned for inherited properties in such cases.

I've included a few commits which clean up some mostly formatting and consistency issues in the various property command docs. The third commit adds a missing DIRECTORY keyword in the signature of set_tests_properties(), and that keyword is new for CMake 3.28. Therefore, I've marked this MR to be back ported to the release branch. All changes are doc-only, and they bring the docs into line with the actual behavior.

Backport: release