[DOCS] Edit validation section of dynamic templates docs

[liebeslied]

A few changes to try to make this documentation clearer.

honestly I'm still not sure I understand this paragraph, and therefore whether my rewording accurately rephrases it. Is it saying that any fields matching the template have to be indexed as string types unless using match_mapping_type? Clarifications welcome!

If no match_mapping_type has been specified then if the template is valid with one predefined mapping type then the mapping snippet is considered valid. However if at index time a field that matches with the template is indexed as a different type then an validation error will occur at index time instead. For example configuring a dynamic template with no match_mapping_type is considered valid as string type, but at index time a field that matches with the dynamic template is indexed as a long, then at index time a validation error may still occur.

[elasticmachine]

Pinging @elastic/es-docs (>docs)

[elasticmachine]

Pinging @elastic/es-search (:Search/Mapping)

[jrodewig]

LGTM. Thanks for your contribution @liebeslied.

I'm going to incorporate a few suggestions, but your improvements made this much more readable.

[jrodewig]

@elasticmachine test this please

[jrodewig]

@elasticmachine test this please

[jrodewig]

Backport commits

master 62e2778
7.x dc12a68
7.8 3f852e1
7.7 8c6ada2

[liebeslied]

@jrodewig nice, thank you :)