Clarify docs.

Remove reference to ERR_TXT_MALLOCED in the error library as that is
only used internally. Indicate that returned error data must not be
freed.
(cherry picked from commit f2d678e6e89b6508147086610e985d4e8416e867)

doc/crypto/ERR_get_error.pod

@@ -52,8 +52,11 @@ ERR_get_error_line_data(), ERR_peek_error_line_data() and
 ERR_get_last_error_line_data() store additional data and flags
 associated with the error code in *B<data>
 and *B<flags>, unless these are B<NULL>. *B<data> contains a string
-if *B<flags>&B<ERR_TXT_STRING>. If it has been allocated by OPENSSL_malloc(),
-*B<flags>&B<ERR_TXT_MALLOCED> is true.
+if *B<flags>&B<ERR_TXT_STRING> is true. 
+
+An application B<MUST NOT> free the *B<data> pointer (or any other pointers
+returned by these functions) with OPENSSL_free() as freeing is handled
+automatically by the error library.
 
 =head1 RETURN VALUES