KMainWindow documentation fixes
This MR:
- makes several language corrections
- applies more Doxygen commands to make everything explicit, especially brief
- makes extensive use of the see command for easy reference
- fixes broken API links
- explicitly refers to Qt API whenever it's mentioned
- changes nonspecific parameter name number and f into clearer numberOfInstances and flags
This is still missing more examples and some squashing.
Edited by Thiago Sueto