src/msgb.c: fix Doxygen documentation for msgb_printf()

This change fixes the following Doxygen warnings:

  src/msgb.c:479: warning: argument 'msg' of command @param is not
                  found in the argument list of
                  msgb_printf(struct msgb *msgb, const char *format,...)
  core/msgb.h:708: warning: The following parameters of
                   msgb_printf(struct msgb *msgb, const char *format,...)
                   are not documented:
                     parameter 'msgb'
                     parameter 'format'

As a bonus, it dot-terminates all sentences.

Change-Id: Ib708664336eef06f748d408ae02a13c754b6647a
diff --git a/src/msgb.c b/src/msgb.c
index 3902f6e..ce7654a 100644
--- a/src/msgb.c
+++ b/src/msgb.c
@@ -476,8 +476,9 @@
 
 
 /*! Print a string to the end of message buffer.
- * \param[in] msg message buffer
- * \returns 0 on success, -EINVAL on error
+ * \param[in] msgb message buffer.
+ * \param[in] format format string.
+ * \returns 0 on success, -EINVAL on error.
  *
  * The resulting string is printed to the msgb without a trailing nul
  * character. A nul following the data tail may be written as an implementation