sql/builtins: update Info for pg_get_viewdef

[ZhouXing19]

The pg_builtin func pg_get_viewdef was updated from a no-op to an actual
function long time ago, but the info field is still notUsableInfo, which
made the doc to miss it by mistake. This PR is to update the info, and let it
be recorded in functions.md.

Release justification: bug fix, update a builtin's visibility in docs.
Release note: none

[cockroach-teamcity]

This change is 

[rafiss]

great, thanks!

[ZhouXing19]

TFTR!
bors r+

[craig]

Build failed (retrying...):

• Bazel Essential CI (Cockroach)

[craig]

Build succeeded:

• Bazel Essential CI (Cockroach)

[blathers-crl]

Encountered an error creating backports. Some common things that can go wrong:

1. The backport branch might have already existed.
2. There was a merge conflict.
3. The backport branch contained merge commits.

You might need to create your backport manually using the backport tool.

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error creating merge commit from eca9bc9 to blathers/backport-release-21.2-86579: POST https://api.github.com/repos/cockroachdb/cockroach/merges: 409 Merge conflict []

you may need to manually resolve merge conflicts with the backport tool.

Backport to branch 21.2.x failed. See errors above.

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error creating merge commit from eca9bc9 to blathers/backport-release-22.1-86579: POST https://api.github.com/repos/cockroachdb/cockroach/merges: 409 Merge conflict []

you may need to manually resolve merge conflicts with the backport tool.

Backport to branch 22.1.x failed. See errors above.

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