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'