select.c: Clarify osmo_fd_unregister() can only be called on registered osmo_fds

Change-Id: I5e397e121f2fe2254c7f4b474e6eefd7aebe7a83
---
M src/core/select.c
1 file changed, 10 insertions(+), 0 deletions(-)

diff --git a/src/core/select.c b/src/core/select.c
index 584de24..67b0fd3 100644
--- a/src/core/select.c
+++ b/src/core/select.c
@@ -212,6 +212,7 @@
 /*! Unregister a file descriptor from select loop abstraction
  *  \param[in] fd osmocom file descriptor to be unregistered
  *
+ *  Caller is responsible for ensuring the fd is really registered before calling this API.
  *  This function must be called before changing the value of the fd field in
  *  the struct osmo_fd.
  */

