[3.6] Clarify what --enable-optimizations does (GH-1847) (GH-2039)

(cherry picked from commit b4e5fee6f5bcc50500ea6261a22021db58955b55)
diff --git a/configure.ac b/configure.ac
index 146ae22..9620067 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1284,7 +1284,7 @@
 AC_SUBST(DEF_MAKE_RULE)
 Py_OPT='false'
 AC_MSG_CHECKING(for --enable-optimizations)
-AC_ARG_ENABLE(optimizations, AS_HELP_STRING([--enable-optimizations], [Enable expensive optimizations (PGO, etc).  Disabled by default.]),
+AC_ARG_ENABLE(optimizations, AS_HELP_STRING([--enable-optimizations], [Enable expensive, stable optimizations (PGO, etc).  Disabled by default.]),
 [
 if test "$enableval" != no
 then
@@ -5452,7 +5452,7 @@
 if test "$Py_OPT" = 'false' -a "$Py_DEBUG" != 'true'; then
     echo "" >&AS_MESSAGE_FD
     echo "" >&AS_MESSAGE_FD
-    echo "If you want a release build with all optimizations active (LTO, PGO, etc)," >&AS_MESSAGE_FD
+    echo "If you want a release build with all stable optimizations active (PGO, etc)," >&AS_MESSAGE_FD
     echo "please run ./configure --enable-optimizations" >&AS_MESSAGE_FD
     echo "" >&AS_MESSAGE_FD
     echo "" >&AS_MESSAGE_FD