gh-146642: Clarify -W regex requires at least two characters

The documentation for the -W option's message and module fields
states that a value starting and ending with a forward slash is
treated as a regular expression. However, the implementation
requires the value to be at least two characters long (so a bare
"/" is treated as a literal string, not a regex). Update the docs
in both cmdline.rst and warnings.rst to mention this requirement.

Author: WYSIATI

Doc/library/warnings.rst

@@ -159,9 +159,9 @@ the disposition of the match.  Each entry is a tuple of the form (*action*,
 
 * *message* is a string containing a regular expression that the start of
   the warning message must match, case-insensitively.  In :option:`-W` and
-  :envvar:`PYTHONWARNINGS`, if *message* starts and ends with a forward slash
-  (``/``), it specifies a regular expression as above;
-  otherwise it is a literal string that the start of the
+  :envvar:`PYTHONWARNINGS`, if *message* is at least two characters long and
+  starts and ends with a forward slash (``/``), it specifies a regular
+  expression as above; otherwise it is a literal string that the start of the
   warning message must match (case-insensitively), ignoring any whitespace at
   the start or end of *message*.
 
@@ -170,9 +170,9 @@ the disposition of the match.  Each entry is a tuple of the form (*action*,
 
 * *module* is a string containing a regular expression that the start of the
   fully qualified module name must match, case-sensitively.  In :option:`-W` and
-  :envvar:`PYTHONWARNINGS`, if *module* starts and ends with a forward slash
-  (``/``), it specifies a regular expression as above;
-  otherwise it is a literal string that the
+  :envvar:`PYTHONWARNINGS`, if *module* is at least two characters long and
+  starts and ends with a forward slash (``/``), it specifies a regular
+  expression as above; otherwise it is a literal string that the
   fully qualified module name must be equal to (case-sensitively), ignoring any
   whitespace at the start or end of *module*.
 

Doc/using/cmdline.rst

@@ -481,8 +481,9 @@ Miscellaneous options
 
    The *message* field must match the start of the warning message;
    this match is case-insensitive.
-   If it starts and ends with a forward slash (``/``), it specifies
-   a regular expression, otherwise it specifies a literal string.
+   If it is at least two characters long and starts and ends with a forward
+   slash (``/``), it specifies a regular expression, otherwise it specifies a
+   literal string.
 
    The *category* field matches the warning category
    (ex: ``DeprecationWarning``). This must be a class name; the match test
@@ -491,10 +492,10 @@ Miscellaneous options
 
    The *module* field matches the (fully qualified) module name; this match is
    case-sensitive.
-   If it starts and ends with a forward slash (``/``), it specifies
-   a regular expression that the start of the fully qualified module name
-   must match, otherwise it specifies a literal string that the fully
-   qualified module name must be equal to.
+   If it is at least two characters long and starts and ends with a forward
+   slash (``/``), it specifies a regular expression that the start of the fully
+   qualified module name must match, otherwise it specifies a literal string
+   that the fully qualified module name must be equal to.
 
    The *lineno* field matches the line number, where zero matches all line
    numbers and is thus equivalent to an omitted line number.