Help: Explain build/install-tree include dirs in more places (#14946)

Explain how to use $<BUILD_INTERFACE> and $<INSTALL_INTERFACE> directly
in the documentation of the target_include_directories command and
INTERFACE_INCLUDE_DIRECTORIES target property.  Otherwise readers need
to notice the link to the cmake-buildsystem(7) manual and find the
example in that to understand the need for these expressions.

Also fix the explanation in cmake-buildsystem(7) to not claim that
relative paths may be used inside a BUILD_INTERFACE expression.
Merge branch 'doc-include-dir-props' into release
Merge topic 'doc-include-dir-props'

907e422b Help: Explain build/install-tree include dirs in more places (#14946)
Merge branch 'release'
Simplify and clarify credit text and link

Use the name "Kitware" rather than "Kitware, Inc." to simplify the text.
Provide a CMake-specific URL.
Merge branch 'cmake-credits' into release
Merge topic 'cmake-credits'

8da0a75b Simplify and clarify credit text and link
Merge branch 'release'
Simplify and clarify credit text and link

Use the name "Kitware" rather than "Kitware, Inc." to simplify the text.
Provide a CMake-specific URL.
Merge topic 'cmake-credits'

889f2fd8 Simplify and clarify credit text and link
Merge branch 'cmake-credits' into release
Merge branch 'release'
Help: Add placeholder documents for CTest Script variables

Provide a minimal explanation of each variable by simply naming
the corresponding ctest(1) setting and linking to the ctest(1)
manual.
Help: Convert CTest Script variable references to cross-references

In the ctest(1) manual and ctest_start() command documentation, convert
mentions of CTest Script variables to cross-references so they will be
rendered as links.
Merge topic 'doc-ctest-variables'

34c6fd3a Help: Convert CTest Script variable references to cross-references
de710f37 Help: Add placeholder documents for CTest Script variables