doc: clarify osmo_strlcpy() doc

Stating that it 'truncates src' is misleading. Also clarify whether siz
includes the space needed for the terminating NUL.

Change-Id: I01c1a94408b471f7f54576178a60938bf9ee3261
diff --git a/src/utils.c b/src/utils.c
index 02c2443..df806df 100644
--- a/src/utils.c
+++ b/src/utils.c
@@ -343,12 +343,12 @@
 /*! \brief Copy a C-string into a sized buffer
  *  \param[in] src source string
  *  \param[out] dst destination string
- *  \param[in] siz size of the \a dst string
- *  \returns length of source string
+ *  \param[in] siz size of the \a dst buffer
+ *  \returns length of \a src
  *
- *  Copies up to \a siz characters from \a src to \a dst, but ensures
- *  that the last character of \a dst is always a NUL character.  May
- *  truncate \a src to do achieve this.
+ * Copy at most \a siz bytes from \a src to \a dst, ensuring that the result is
+ * NUL terminated. The NUL character is included in \a siz, i.e. passing the
+ * actual sizeof(*dst) is correct.
  */
 size_t osmo_strlcpy(char *dst, const char *src, size_t siz)
 {