Change formulation for space inside <mtext> elements

[MartheGjelstad]

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.

[TimArborealis]

Sounds good and clear to me.