Change formulation for space inside <mtext> elements

Should we clarify the markup of space in `<mtext>` elements? 

Suggestion for new formulation: If you press spacebar at the beginning or end of an `<mtext>` element, there will be no visible space. Therefore, you need to use the "four-per-em-space". However, between words in an `<mtext>` element, the spacebar works. 