added some more Glibc desc's

git-svn-id: http://svn.linuxfromscratch.org/LFS/trunk/BOOK@1884 4aa44e1e-78dd-0310-a6d2-fbcd4c07a689

appendixa/glibc-desc.xml

@@ -170,10 +170,11 @@ lines.</para></sect4>
 <para>libm is the mathematical library.</para></sect4>
 
 <sect4><title>libmcheck</title>
-<para>No description is currently available.</para></sect4>
+<para>libmcheck contains code run at boot.</para></sect4>
 
 <sect4><title>libmemusage</title>
-<para>No description is currently available.</para></sect4>
+<para>libmemusage is used by memusage to help collect information about the
+memory usage of a program.</para></sect4>
 
 <sect4><title>libnsl, libnsl_p</title>
 <para>No description is currently available.</para></sect4>