Macro-by-Examples' "Metavariables" section should link to the "Follow-set Ambiguity Restriction" section

[bzm3r]

Currently, someone using the reference to figure out why a macro-by-example isn't working will probably look at the Metavariables section to see how their syntax usage matches with the language definitions provided. However, the syntax of metavariables is only partially defined by the language's syntax; it is additionally constrained by the Follow-set Ambiguity Restrictions.

Could a link be placed prominently in the "Metavariables" section to the "Follow-set Ambiguity Restrictions" section, so that people are less likely to miss this?

(I can submit a PR for this, if it is judged to be worthwhile.)