Add an explanatory comment to lib/libc/gen/isnan.c about the fix to make

static linking with libc and libm work.

Requested by:	jilles
MFC after:	1 week
X-MFC-With:	242879

lib/libc/gen/isnan.c

@@ -33,6 +33,11 @@
 /*
  * XXX These routines belong in libm, but they must remain in libc for
  *     binary compat until we can bump libm's major version number.
+ *
+ * Note this only applies to the dynamic versions of libm and libc, so
+ * for the static and profiled versions we stub out the definitions.
+ * Otherwise you cannot link statically to libm and libc at the same
+ * time, when calling both functions.
  */
 
 #ifdef PIC