Docfix: ignore_above uses string length, not utf-8
Docfix: ignore_above uses string length, not utf-8

ignore_above is used to guard against the lucene limitation
that a term cannot exceed 32766 bytes.

However, the implementation just used the character count, which
doesn't take into account the fact that some characters have
multi-byte utf-8 encodings.

This commit updates the docs to make this relationship clear.

Closes #11563

fubuki/elasticsearch,fubuki/elasticsearch,fubuki/elasticsearch,fubuki/elasticsearch,fubuki/elasticsearch,fubuki/elasticsearch