BUG#36381815 step 4.1: Update readme file and header guard in mysql_utils, after step 4

Part of: Bug#36381815: Factor out generic functionality from the mysql_binlog_event library

Update the readme file for the mysql_utils library after the previous
step.  Fix the header guard name in nodiscard.h.

(We don't squash this into step 3, in order to keep step e
reproducible with scripts.)

Change-Id: I0e56500b32fe6a7ce91df1c1997c82e8b6e654e6

Author: Sven Sandberg

libs/mysql/utils/nodiscard.h

@@ -21,8 +21,8 @@
    along with this program; if not, write to the Free Software
    Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301  USA */
 
-#ifndef MYSQL_BINLOG_UTILS_NODISCARD_H
-#define MYSQL_BINLOG_UTILS_NODISCARD_H
+#ifndef MYSQL_UTILS_NODISCARD_H
+#define MYSQL_UTILS_NODISCARD_H
 
 /// The function attribute [[NODISCARD]] is a replacement for
 /// [[nodiscard]] to workaround a gcc bug.
@@ -47,4 +47,4 @@
 #define NODISCARD nodiscard
 #endif
 
-#endif  // MYSQL_BINLOG_UTILS_NODISCARD_H
+#endif  // MYSQL_UTILS_NODISCARD_H

libs/mysql/utils/readme.md

@@ -35,8 +35,13 @@ Code documentation: @ref GroupLibsMysqlUtils.
 
 This is a header-only library, containing various utility functions, such as:
 
-- macro deprecating a header
-- definition of Error - base class for (C++) error handling
-- enumeration utilities: to_underlying (-std=C++<23>), to_enumeration
-- template utilities: Is_specialization
-- bit operations
+- bounded_arithmetic.h: multiplication and addition, capped at a max value
+- concat.h: vararg function to concatenate many values to a string
+- deprecate_header.h: macro deprecating a header
+- enumeration_utils.h: to_underlying (backported from C++23), to_enumeration
+- error.h: definition of Error - base class for (C++) error handling
+- is_specialization.h: utility to determine if a template is a specialization
+  of another.
+- nodiscard.h: replacement for [[nodiscard]] that works around gcc bug
+  https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84476
+- return_status.h: enum { ok, error }