doc: Clarify msg when doc/api/cli.md not updated
Checklist
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commit message follows commit guidelines
Affected core subsystem(s)
build
The message printed out when the doc/api/cli.md file had not had the REPLACEME tags replaced could do with being made clearer. This PR updates it to indicate which the top level Makefile is checking for.
Cc: @bnoordhuis who put in the original message.