docs(configuraExplain the externals string syntax specifying externalsType (#6316)

* Update externals.mdx

* explain more for externalsType

* Apply suggestions from code review

Co-authored-by: Sam Chen <[email protected]>

* change based on suggestions

* Apply suggestions from code review

* Update externals.mdx

Co-authored-by: Sam Chen <[email protected]>

Author: tanhauhau