FindKDE3: Update documentation

- This syncs the module documentation with other similar find modules.
- Added note about KDE version 3 being obsolete and newer KDE version
  should be used. A link to upstream docs is added.
- Fixed typo "SKIP_AUTOMAKE" to "SKIP_AUTOMOC".
- Documented commands. Instead of mentioning "macros", this now uses the
  more general wording "commands" and lists them similar to other
  documentation pages.
- Lowercase style used for commands.
- Reworded descriptions to match the implementations a bit.