docs update: tie CAP_SUPPORTS_TRANSACTIONAL_OPS to addCall

The previous documentation was too vague and caused confusion.  Clarify
that `CAPABILITY_SUPPORTS_TRANSACTIONAL_OPERATIONS` is exclusively for
apps using the TelecomManager#addCall

Flag: EXEMPT docs update
Bug: 416832675
Test: verified UI render not broken + build
Change-Id: Ice3b9b264358d1dd6fe74fdfe2c18ba99cce9818