Clarify contract of AlphaNum as only a function parameter type

PiperOrigin-RevId: 502901875
Change-Id: I1c8c097e5c81a9e413692109ebfe0d0b24f50f2e

absl/strings/str_cat.h

@@ -242,8 +242,10 @@ struct Dec {
 // -----------------------------------------------------------------------------
 // -----------------------------------------------------------------------------
 //
 //
 // The `AlphaNum` class acts as the main parameter type for `StrCat()` and
 // The `AlphaNum` class acts as the main parameter type for `StrCat()` and
-// `StrAppend()`, providing efficient conversion of numeric, boolean, and
-// hexadecimal values (through the `Hex` type) into strings.
+// `StrAppend()`, providing efficient conversion of numeric, boolean, decimal,
+// and hexadecimal values (through the `Dec` and `Hex` types) into strings.
+// `AlphaNum` should only be used as a function parameter. Do not instantiate
+//  `AlphaNum` directly as a stack variable.
 
 
 class AlphaNum {
 class AlphaNum {
  public:
  public: