docs: missing 'repeatable' in @goExtraField directive

[alexandear]

According to the GraphQL spec, directives that are used more than once must be defined as repeatable:

GraphQL allows directives that are defined as repeatable to be used more than once on the definition they apply to, possibly with different arguments. In contrast, if a directive is not repeatable, then only one occurrence of it is allowed per location.

Fixes #3145

I have:

• Updated any relevant documentation (see docs)

[coveralls]

coverage: 74.624%. remained the same
when pulling 63c01b1 on alexandear:docs-fix-def-go-extra-field
into 85459a3 on 99designs:master.