Improve documentation for module APIs

Improve documentation for module APIs

Previously, it was not made clear to plugin developers why they might
wish to implement a module API. This commit adds documentation which
describes how module APIs can be useful, in the same section as the
other qml type registration explanatory documentation.

Task-number: QTBUG-25731
Change-Id: I2350ce8e5ae90598a65295fb2afb7dbd23eb7da6
Reviewed-by: Bea Lam <bea.lam@nokia.com>