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

Doc/library/string.rst

@@ -428,12 +428,13 @@ character of ``'0'`` with an *alignment* type of ``'='``.
    Preceding the *width* field by ``'0'`` no longer affects the default
    alignment for strings.
 
-The *precision* is a decimal number indicating how many digits should be
-displayed after the decimal point for a floating point value formatted with
-``'f'`` and ``'F'``, or before and after the decimal point for a floating point
-value formatted with ``'g'`` or ``'G'``.  For non-number types the field
+The *precision* is a decimal integer indicating how many digits should be
+displayed after the decimal point for presentation types
+``'f'`` and ``'F'``, or before and after the decimal point for presentation
+types ``'g'`` or ``'G'``.  For string presentation types the field
 indicates the maximum field size - in other words, how many characters will be
-used from the field content. The *precision* is not allowed for integer values.
+used from the field content.  The *precision* is not allowed for integer
+presentation types.
 
 Finally, the *type* determines how the data should be presented.