Make help texts more descriptive in Metaplugin

This patch makes help texts of configuration parameters of
metaplugin.ini more descriptive. Comments in metaplugin.ini
are also improved.

Change-Id: I1fac888ecc223a7199cc9c9aeb603b7493f22171
Closes-Bug: #1283930
Merge "Make help texts more descriptive in Metaplugin"