docs: Improve wording in documentation of g_unix_signal_source_new()

- Insert missing word "from".

- Remove space between function name and "()" so syntax highlighting
  can recognise it as a function.

- Avoid "you"/"your" when discussing the reentrancy issues of regular
  UNIX signal handlers, because it gives the false impression that
  these issues are applicable to g_unix_signal_source_new().

Unrelated:

- Fix missing space in documentation of g_signal_new_class_handler().

glib/glib-unix.c

@@ -205,11 +205,11 @@ g_unix_set_fd_nonblocking (gint       fd,
  *
  * For example, an effective use of this function is to handle `SIGTERM`
  * cleanly; flushing any outstanding files, and then calling
- * g_main_loop_quit ().  It is not safe to do any of this a regular
+ * g_main_loop_quit().  It is not safe to do any of this from a regular
- * UNIX signal handler; your handler may be invoked while malloc() or
+ * UNIX signal handler; such a handler may be invoked while malloc() or
- * another library function is running, causing reentrancy if you
+ * another library function is running, causing reentrancy issues if the
- * attempt to use it from the handler.  None of the GLib/GObject API
+ * handler attempts to use those functions.  None of the GLib/GObject
- * is safe against this kind of reentrancy.
+ * API is safe against this kind of reentrancy.
  *
  * The interaction of this source when combined with native UNIX
  * functions like sigprocmask() is not defined.

gobject/gsignal.c

@@ -1554,7 +1554,7 @@ g_signal_new (const gchar	 *signal_name,
  * an object definition, instead the function pointer is passed
  * directly and can be overridden by derived classes with
  * g_signal_override_class_closure() or
- * g_signal_override_class_handler()and chained to with
+ * g_signal_override_class_handler() and chained to with
  * g_signal_chain_from_overridden() or
  * g_signal_chain_from_overridden_handler().
  *