man/io_uring_register_files.3: clarify why registered files exist
@@ -38,6 +38,13 @@ and later.
Registering a file table is a prerequisite for using any request that uses
direct descriptors.
+Registered files have less overhead per operation than normal files. This
+is due to the kernel grabbing a reference count on a file when an operation
+begins, and dropping it when it's done. When the process file table is
+shared, for example if the process has ever created any threads, then this
+cost goes up even more. Using registered files reduces the overhead of
+file reference management across requests that operate on a file.
+
.SH RETURN VALUE
On success
.BR io_uring_register_files (3)
