make: config: more comment

Author: bradjc

Configuration.mk

@@ -193,7 +193,8 @@ override CPPFLAGS_PIC += \
       -fPIC
 
 ifneq ($(PICOLIB),)
-  # Use picolib
+  # Use picolib for libc. We need to include the `picolib-tock` library which
+  # maps Tock's system-level functions to the names that picolib expects.
   EXTERN_LIBS += $(TOCK_USERLAND_BASE_DIR)/picolib-tock
 endif