Corrections to format precision description. (GH-31291)

* `precision` field is a decimal integer
  * clarify that stated limitations are on presentation type
    rather than input value type.  Especially misleading is
    "precision is not allowed for integer values", since integer
    value input to a format like `.1f` is fine.
  * regarding max field size, replace "non-number" with "string",
    which is the only non-numeric presentation type

Automerge-Triggered-By: GH:ericvsmith