FindFLEX: Update documentation

- Updated and synced module documentation with other similar find
  modules.
- Used lowercase style for commands across the docs.
- Commands documented with indentation.
- Command arguments described as a list separately.
- Used "commands" instead of "macros".
- Extended examples section.
- Added example showing how to use found Flex library by creating an
  imported target inside a project. Fixes: #18718