CheckVariableExists: Update documentation

- Added intro code block showing how to include this module.
- Added examples section.
- Added missing CMAKE_REQUIRED_LINK_DIRECTORIES variable (this module
  can also use it as of CMake 3.31).
- Added a note that this is a linker only check and it doesn't check
  whether the variable is also declared in headers, unlike
  check_symbol_exists().
- Added "See Also" section.