read: add IFS= and -r to while loop example (#23332)

Without `IFS=`, the `line` variable will have leading
and trailing whitespace stripped. Without `-r`, each
line will have backslashes interpreted as escape
characters. This is surprising behavior when trying
to perform an action on each line, and nearly all
examples of this pattern I've seen in blogs etc.
include IFS= and -r for this reason.

Explanation of IFS= and -r:
https://unix.stackexchange.com/questions/209123/understanding-ifs-read-r-line

Shell check lint for this:
https://www.shellcheck.net/wiki/SC2013

Co-authored-by: jxu <7989982+jxu@users.noreply.github.com>

pages/common/read.md

@@ -31,6 +31,6 @@
 
 `read -s {{variable}}`
 
-- Read `stdin` and perform an action on every line:
+- Perform an action on each line of a command's output:
 
-`cat {{/dev/stdin|path/to/file|...}} | while read line; do {{echo|ls|rm|...}} "$line"; done`
+`{{command}} | while IFS= read -r line; do {{echo|ls|rm|...}} "$line"; done`