fix api doc of osmo_identifier_sanitize_buf()

The implementation cleary replaces sep_chars instead of keeping them.
Adjust the doc to clarify that.

It seems to me that the intention was to *allow* sep_chars instead of
replacing them, especially when looking at sep_chars of
osmo_separated_identifiers_valid(). But we're stuck with that now.

Change-Id: Ia842a7eeb46b49ee364be55c1194e463ebcaf392
diff --git a/src/utils.c b/src/utils.c
index f5896c4..721c34a 100644
--- a/src/utils.c
+++ b/src/utils.c
@@ -687,7 +687,7 @@
  * To guarantee passing osmo_separated_identifiers_valid(), replace_with must not itself be an illegal character. If in
  * doubt, use '-'.
  * \param[inout] str  Identifier to sanitize, must be nul terminated and in a writable buffer.
- * \param[in] sep_chars  Additional characters that are allowed besides osmo_identifier_illegal_chars.
+ * \param[in] sep_chars  Additional characters that are to be replaced besides osmo_identifier_illegal_chars.
  * \param[in] replace_with  Replace any illegal characters with this character.
  */
 void osmo_identifier_sanitize_buf(char *str, const char *sep_chars, char replace_with)