Move documentation from function declarations to definitions

Uses a perl script to move it (scripts/movedocs.pl)

src/nvim/os/input.c

@@ -124,6 +124,10 @@ void os_breakcheck()
     fill_input_buf(false);
 }
 
+/// Test whether a file descriptor refers to a terminal.
+///
+/// @param fd File descriptor.
+/// @return `true` if file descriptor refers to a terminal.
 bool os_isatty(int fd)
 {
     return uv_guess_handle(fd) == UV_TTY;