minor rewording in ceil(), ceiling() and floor() function descriptions

The following documentation comment has been logged on the website:

Page: https://www.postgresql.org/docs/12/functions-math.html
Description:

Proposed rewording for ceil() and ceiling(): "smallest integer..."
Proposed rewording for floor(): "greatest integer..."

Rationale: the proposed rewording is more aligned to their respective
mathematical definitions

References:

- https://proofwiki.org/wiki/Definition:Ceiling_Function
- https://proofwiki.org/wiki/Definition:Floor_Function