Document failure modes of xcb_connect*() functions

Documentation was previously unclear that these always return a non-NULL
pointer, and that callers need to check it for error values, instead of
checking for a NULL return value.

Triggered by having to dig through code to answer a user's question on
the #xcb irc channel, since neither of us found it covered in the docs.

Signed-off-by: Alan Coopersmith <alan.coopersmith@oracle.com>
Reviewed-by: Uli Schlachter <psychon@znc.in>

src/xcb.h

@@ -453,7 +453,8 @@ int xcb_get_file_descriptor(xcb_connection_t *c);
  * Some errors that occur in the context of an xcb_connection_t
  * are unrecoverable. When such an error occurs, the
  * connection is shut down and further operations on the
- * xcb_connection_t have no effect.
+ * xcb_connection_t have no effect, but memory will not be freed until
+ * xcb_disconnect() is called on the xcb_connection_t.
  *
  * @return XCB_CONN_ERROR, because of socket errors, pipe errors or other stream errors.
  * @return XCB_CONN_CLOSED_EXT_NOTSUPPORTED, when extension not supported.
@@ -475,6 +476,11 @@ int xcb_connection_has_error(xcb_connection_t *c);
  * bidirectionally connected to an X server. If the connection
  * should be unauthenticated, @p auth_info must be @c
  * NULL.
+ *
+ * Always returns a non-NULL pointer to a xcb_connection_t, even on failure.
+ * Callers need to use xcb_connection_has_error() to check for failure.
+ * When finished, use xcb_disconnect() to close the connection and free
+ * the structure.
  */
 xcb_connection_t *xcb_connect_to_fd(int fd, xcb_auth_info_t *auth_info);
 
@@ -520,6 +526,11 @@ int xcb_parse_display(const char *name, char **host, int *display, int *screen);
  * variable. If a particular screen on that server is preferred, the
  * int pointed to by @p screenp (if not @c NULL) will be set to that
  * screen; otherwise the screen will be set to 0.
+ *
+ * Always returns a non-NULL pointer to a xcb_connection_t, even on failure.
+ * Callers need to use xcb_connection_has_error() to check for failure.
+ * When finished, use xcb_disconnect() to close the connection and free
+ * the structure.
  */
 xcb_connection_t *xcb_connect(const char *displayname, int *screenp);
 
@@ -534,6 +545,11 @@ xcb_connection_t *xcb_connect(const char *displayname, int *screenp);
  * authorization @p auth. If a particular screen on that server is
  * preferred, the int pointed to by @p screenp (if not @c NULL) will
  * be set to that screen; otherwise @p screenp will be set to 0.
+ *
+ * Always returns a non-NULL pointer to a xcb_connection_t, even on failure.
+ * Callers need to use xcb_connection_has_error() to check for failure.
+ * When finished, use xcb_disconnect() to close the connection and free
+ * the structure.
  */
 xcb_connection_t *xcb_connect_to_display_with_auth_info(const char *display, xcb_auth_info_t *auth, int *screen);