Fix QDoc warnings in the Extending Qt Creator Manual

The Extending Qt Creator Manual will be published at doc.qt.io (in addition to the current snapshot release), so it would be good to fix at least the obvious QDoc warnings, such as wrong document signatures in \fn commands, wrong QDoc commands, and other QDoc formatting issues.

Fix the warnings for the following libs and plugins and hide the rest:

• src/libs/aggregation 
• src/libs/extensionsystem

After this is done, start documenting the rest in the order of importance (will create separate tasks for this):

• src/plugins/coreplugin/actionmanager
• src/plugins/coreplugin
• src/plugins/vcsbase
• src/plugins/projectexplorer (undergoing heavy changes)
• src/plugins/texteditor
• src/libs/utils

The priority of documenting the rest will be determined case-by-case.