Replace all instances of 'column path' in help messages with 'cell path' (#7063)

* Rewrite all 'column path' instances to 'cell path'

* Minor tweak

crates/nu-command/src/strings/str_/substring.rs

@@ -48,7 +48,7 @@ impl Command for SubCommand {
             .rest(
                 "rest",
                 SyntaxShape::CellPath,
-                "optionally substring text by column paths",
+                "For a data structure input, turn strings at the given cell paths into substrings",
             )
     }