nl: added read from stdout and number blank lines. (#1903)

pages/linux/nl.md

@@ -2,14 +2,22 @@
 
 > A utility for numbering lines, either from a file, or from standard input.
 
-- Number lines in a file:
+- Number non-blank lines in a file:
 
 `nl {{file}}`
 
+- Read from standard output:
+
+`cat {{file}} | nl {{options}} -`
+
 - Number only the lines with printable text:
 
 `nl -t {{file}}`
 
+- Number all lines including blank lines:
+
+`nl -b a {{file}}`
+
 - Number only the body lines that match a basic regular expression (BRE) pattern:
 
 `nl -b p'FooBar[0-9]' {{file}}`