lint W47: clarify empty description message

[rjbou]

trafficstars

~Specify that descr section is empty instead of description~ remove descr note in order to deprecate it fix #4989

-warning 47: Synopsis (or description first line) should start with a capital and not end with a dot
+warning 47: Synopsis  should start with a capital and not end with a dot

[rjbou]

from dev meeting: We need to check synopsis & description usage. synopsis must not be empty, and description is optional. it's descr that is optional -> add a deprectaed message when encountered (in order to remove it)

[rjbou]

proposal: in this PR, we remove the descr reference, and in another, we deprecate and clean-up OpamFile.Descr usage

[dra27]

This just needs:

• Changes entry updating, as it got out of sync with the PR
• An issue to be opened to track adding a new lint warning for missing synopsis

[kit-ty-kate]

Thanks!

[kit-ty-kate]

The followup PR to restore the missing lint is at https://github.com/ocaml/opam/pull/5442