Improve wording of str replace help messages (#7708)

# Description

See title.

# User-Facing Changes

See title.

# Tests + Formatting

Don't forget to add tests that cover your changes.

Make sure you've run and fixed any issues with these commands:

- `cargo fmt --all -- --check` to check standard code formatting (`cargo
fmt --all` applies these changes)
- `cargo clippy --workspace -- -D warnings -D clippy::unwrap_used -A
clippy::needless_collect` to check that you're using the standard code
style
- `cargo test --workspace` to check that all tests pass

# After Submitting

If your PR had any user-facing changes, update [the
documentation](https://github.com/nushell/nushell.github.io) after the
PR is merged, if necessary. This will help us keep the docs up to date.

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

@@ -36,21 +36,21 @@ impl Command for SubCommand {
             .input_output_types(vec![(Type::String, Type::String)])
             .vectorizes_over_list(true)
             .required("find", SyntaxShape::String, "the pattern to find")
-            .required("replace", SyntaxShape::String, "the replacement pattern")
+            .required("replace", SyntaxShape::String, "the replacement string")
             .rest(
                 "rest",
                 SyntaxShape::CellPath,
                 "For a data structure input, operate on strings at the given cell paths",
             )
-            .switch("all", "replace all occurrences of find string", Some('a'))
+            .switch("all", "replace all occurrences of the pattern", Some('a'))
             .switch(
                 "no-expand",
-                "do not expand the replace parameter as a regular expression",
+                "do not expand capture groups (like $name) in the replacement string",
                 Some('n'),
             )
             .switch(
                 "string",
-                "do not use regular expressions for string find and replace",
+                "match the pattern as a substring of the input, instead of a regular expression",
                 Some('s'),
             )
             .category(Category::Strings)