INTERVAL: documentation updated

(cherry picked from commit 6a5f8288624a37d67939ad4af39eaac4b732e171)

Conflicts:
	src/bias/MetaD.cpp
METAD: documenation updated to say that hills height is
rescaled in case of WTMetaD.

(cherry picked from commit ee3c0a9a8a5e0050695e123e93c7fbccc867c9bd)