Add note for authors about preventing whitespace trimming and collapsing in <ms> elements #290

The current MathML specifications contain the following note:

Like all token elements, ms does trim and collapse whitespace in its content according to the rules of Section 2.1.7 Collapsing Whitespace in Input, so whitespace intended to remain in the content should be encoded as described in that section.

A similar note should be added to MathML Core, since this differs from the <pre> element in HTML.