Doc: Fix 'No output generated for property' documentation warnings

qqmlwebchannel.cpp documents only the WebChannel QML type. Remove
superfluous \property documentation not tied to any documented
class, and move \qmlproperty documentation comments to be adjacent
to the implementation.

Fixes

qqmlwebchannel.cpp:242: warning: No output generated for property
'QQmlWebChannel::transports' because 'QQmlWebChannel' is undocumented

qqmlwebchannel.cpp:173: warning: No output generated for property
'QQmlWebChannel::registeredObjects' because 'QQmlWebChannel' is
undocumented

Pick-to: 6.12 6.11
Task-number: QTBUG-141665
Change-Id: I65aa8b0e65c9c63692605a6529a0ef105fd24ae0
Reviewed-by: Paul Wicking <paul.wicking@qt.io>