diagnostics: add comments about global_dc

No functional change intended.

gcc/ChangeLog:
	* diagnostic-core.h: Add comments making clear that these
	functions implicitly use global_dc.

Signed-off-by: David Malcolm <dmalcolm@redhat.com>

gcc/diagnostic-core.h

 /* Declarations of core diagnostic functionality for code that does
    not need to deal with diagnostic contexts or diagnostic info
-   structures.
+   structures.  These functions implicitly use global_dc.
    Copyright (C) 1998-2025 Free Software Foundation, Inc.
 
 This file is part of GCC.
@@ -39,7 +39,7 @@ typedef enum
   DK_ANY,
 } diagnostic_t;
 
-/* RAII-style class for grouping related diagnostics.  */
+/* RAII-style class for grouping related diagnostics within global_dc.  */
 
 class auto_diagnostic_group
 {
@@ -48,7 +48,7 @@ class auto_diagnostic_group
   ~auto_diagnostic_group ();
 };
 
-/* RAII-style class for nesting hierarchical diagnostics.
+/* RAII-style class for nesting hierarchical diagnostics within global_dc.
    Any diagnostics emitted within the lifetime of this object
    will be treated as one level of nesting deeper than diagnostics
    emitted outside the lifetime of the object.  */
@@ -88,6 +88,9 @@ extern const char *progname;
 
 extern const char *trim_filename (const char *);
 
+/* Various functions for emitting diagnostics follow.
+   All of these implicitly use global_dc.  */
+
 /* If we haven't already defined a front-end-specific diagnostics
    style, use the generic one.  */
 #ifndef GCC_DIAG_STYLE