cmake: say 'absolute path' in option descriptions and docs

To not have to guess. Also to sync with autotools, which already uses
this wording.

Also:
- replace the stray term 'folder' with 'directory' for consistency.
- store help text in a temp variable to avoid overly long strings
  (mandatory in CMake <4.2.0 and can't be trivially split), also
  to avoid repeating this string 4 times.

Ref: https://cmake.org/cmake/help/v4.2/command/set.html

Closes #19169

CMake/FindLibrtmp.cmake

@@ -25,8 +25,8 @@
 #
 # Input variables:
 #
-# - `LIBRTMP_INCLUDE_DIR`:   The librtmp include directory.
-# - `LIBRTMP_LIBRARY`:       Path to `librtmp` library.
+# - `LIBRTMP_INCLUDE_DIR`:   Absolute path to librtmp include directory.
+# - `LIBRTMP_LIBRARY`:       Absolute path to `librtmp` library.
 #
 # Result variables:
 #