pages/*: add backticks around I/O streams (#10436)

* as suggested in #10428
* standard input  => `stdin`
* standard output => `stdout`
* standard error  => `stderr`

Co-authored-by: K.B.Dharun Krishna <kbdharunkrishna@gmail.com>

pages/common/batch.md

@@ -4,11 +4,11 @@
 > Service atd (or atrun) should be running for the actual executions.
 > More information: <https://manned.org/batch>.
 
-- Execute commands from standard input (press `Ctrl + D` when done):
+- Execute commands from `stdin` (press `Ctrl + D` when done):
 
 `batch`
 
-- Execute a command from standard input:
+- Execute a command from `stdin`:
 
 `echo "{{./make_db_backup.sh}}" | batch`