Update head/tail docs with regards to surrounding pairs (#2676)

## Checklist

- [/] I have added
[tests](https://www.cursorless.org/docs/contributing/test-case-recorder/)
- [x] I have updated the
[docs](https://github.com/cursorless-dev/cursorless/tree/main/docs) and
[cheatsheet](https://github.com/cursorless-dev/cursorless/tree/main/cursorless-talon/src/cheatsheet)
- [x] I have not broken the cheatsheet

---------

Co-authored-by: Pokey Rule <[email protected]>

Author: AndreasArvidsson

src/cheatsheet/sections/modifiers.py

@@ -64,7 +64,7 @@ def get_modifiers():
             "variations": [
                 {
                     "spokenForm": complex_modifiers["extendThroughStartOf"],
-                    "description": "Extend through start of line",
+                    "description": "Extend through start of line/pair",
                 },
                 {
                     "spokenForm": f"{complex_modifiers['extendThroughStartOf']} <modifier>",
@@ -78,7 +78,7 @@ def get_modifiers():
             "variations": [
                 {
                     "spokenForm": complex_modifiers["extendThroughEndOf"],
-                    "description": "Extend through end of line",
+                    "description": "Extend through end of line/pair",
                 },
                 {
                     "spokenForm": f"{complex_modifiers['extendThroughEndOf']} <modifier>",