Base: Note in pledge(2) man page which promises are extensions

Also add a little "History" section noting that pledge() is an original
OpenBSD invention and that our implementation differs in many ways.

Base/usr/share/man/man2/pledge.md

@@ -27,26 +27,32 @@ If `promises` or `execpromises` is null, the corresponding value is unchanged.
 ## Promises
 
 * `stdio`: Basic I/O, memory allocation, information about self, various non-destructive syscalls
-* `thread`: The POSIX threading API
+* `thread`: The POSIX threading API (\*)
 * `id`: Ability to change UID/GID
 * `tty`: TTY related functionality
 * `proc`: Process and scheduling related functionality
 * `exec`: The [`exec(2)`](exec.md) syscall
 * `unix`: UNIX local domain sockets
 * `inet`: IPv4 domain sockets
-* `accept`: May use [`accept(2)`](accept.md) to accept incoming socket connections on already listening sockets. It also allows [`getsockopt(2)`](getsockopt.md) with `SOL_SOCKET` and `SO_PEERCRED` on local sockets
+* `accept`: May use [`accept(2)`](accept.md) to accept incoming socket connections on already listening sockets. It also allows [`getsockopt(2)`](getsockopt.md) with `SOL_SOCKET` and `SO_PEERCRED` on local sockets (\*)
 * `rpath`: "Read" filesystem access
 * `wpath`: "Write" filesystem access
 * `cpath`: "Create" filesystem access
 * `dpath`: Creating new device files
 * `chown`: Changing file owner/group
 * `fattr`: Changing file attributes/permissions
-* `shared_buffer`: Shared memory buffers
-* `chroot`: The [`chroot(2)`](chroot.md) syscall
+* `shared_buffer`: Shared memory buffers (\*)
+* `chroot`: The [`chroot(2)`](chroot.md) syscall (\*)
 * `video`: May use [`ioctl(2)`](ioctl.md) and [`mmap(2)`](mmap.md) on framebuffer video devices
 
+Promises marked with an asterisk (\*) are SerenityOS specific extensions not supported by the original OpenBSD `pledge()`.
+
 ## Errors
 
 * `EFAULT`: `promises` and/or `execpromises` are not null and not in readable memory.
 * `EINVAL`: One or more invalid promises were specified.
 * `EPERM`: An attempt to increase capabilities was rejected.
+
+## History
+
+The `pledge()` system call was first introduced by OpenBSD. The implementation in SerenityOS differs in many ways and is by no means final.