gmessages: Add IDs for documentation headers

This fixes some broken cross-references between various places in the
logging documentation.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

glib/gmessages.c

@@ -94,7 +94,7 @@
  *  * Color output needed to be supported on the terminal, to make reading
  *    through logs easier.
  *
- * ## Using Structured Logging
+ * ## Using Structured Logging ## {#using-structured-logging}
  *
  * To use structured logging (rather than the old-style logging), either use
  * the g_log_structured() and g_log_structured_array() functions; or define
@@ -104,7 +104,7 @@
  * You do not need to define `G_LOG_USE_STRUCTURED` to use g_log_structured(),
  * but it is a good idea to avoid confusion.
  *
- * ## Log Domains
+ * ## Log Domains ## {#log-domains}
  *
  * Log domains may be used to broadly split up the origins of log messages.
  * Typically, there are one or a few log domains per application or library.
@@ -116,7 +116,7 @@
  * application or library name, optionally followed by a hyphen and a sub-domain
  * name. For example, `bloatpad` or `bloatpad-io`.
  *
- * ## Debug Message Output
+ * ## Debug Message Output ## {#debug-message-output}
  *
  * The default log functions (g_log_default_handler() for the old-style API and
  * g_log_writer_default() for the structured API) both drop debug and
@@ -129,7 +129,7 @@
  * so that developers can re-use the same debugging techniques and tools across
  * projects.
  *
- * ## Testing for Messages
+ * ## Testing for Messages ## {#testing-for-messages}
  *
  * With the old g_log() API, g_test_expect_message() and
  * g_test_assert_expected_messages() could be used in simple cases to check