pread(2) might fail with EBUSY, so document it

PR:		docs/167201
Submitted by:	Kurt Jaeger <fbsd-ports@opsec.eu>
Approved by:	cperciva
MFC after:	3 days

lib/libc/sys/read.2

@@ -164,6 +164,8 @@ argument
 points outside the allocated address space.
 .It Bq Er EIO
 An I/O error occurred while reading from the file system.
+.It Bq Er EBUSY
+Failed to read from a file, e.g. /proc/<pid>/regs while <pid> is not stopped
 .It Bq Er EINTR
 A read from a slow device
 (i.e.\& one that might block for an arbitrary amount of time)