Extra comment to mysql_install_db

Explain why it makes sense to not consider builddir == srcdir directly,
for cases when we do out-of-source builds.

scripts/mysql_install_db.sh

@@ -243,6 +243,8 @@ then
 fi
 if test -n "$srcdir"
 then
+  # In an out-of-source build, builddir is not srcdir. Try to guess where
+  # builddir is by looking for my_print_defaults.
   if test -z "$builddir"
   then
     if test -x "$dirname0/extra/my_print_defaults"