Minor documentation improvements #41

Problem description
Documentation for this API is generally sufficient, but some of the terms could be updated or better defined for clarity and correctness.

For example:
"current subscription status of a phone number" -> "current mobile subscription status identified by the phone number"
"business subscription status" -> "mobile subscription status"
"user" / "subscriber" -> "end user"
"operator" -> "mobile network operator"

Expected action

• Review terminology used in the API description
• Update definitions and descriptions for consistency with other KYC APIs, and to make it clear which actor or entity is involved
• Consider expanding the Commonalities Glossary for terms that do not have an accepted definition

Additional context
None