Add array type hints to minor methods in the Html class
I'm aware that adding these type hints does have the potential of beeing
a breaking change if a caller misuses it. Note that it really is a misuse
in this case because all these parameters are documented as "array" and
nothing else.
I double-checked the usages of all methods I touched and could not find
any caller that does not fulfill the contract of these methods - in other
words, all callers I can find in my local code base (which includes all
major extensions like Echo, Flow, Parsoid, VisualEditor and so on) pass
arrays to these parameters.
I left the main methods openElement, rawElement and so on untouched
because they are called way to often (500 times and more).
Change-Id: I5ca13b26fb08d732ce4cadc4ee3d38314e606fd3