Update Expression.md

Author: CarkHT

docs/teacher/reference/response_area_components/Expression.md

@@ -12,6 +12,23 @@ Calculates the difference between the teacher answer (ans) and the student respo
 
 Compares two symbolic expressions for mathematical equivalence, using SymPy. See [SymPy](https://www.sympy.org/en/index.html.md-button) for further information.
 
+## compareExpressions
+**Input Symbols**
+
+This is a powerful feature for defining a dictionary of accepted symbols. For each symbol, you define:
+
+*    **Symbol:** The LaTeX-rendered symbol (e.g., `$f(x)$`).
+*    **Code:** The machine-readable variable name (e.g., `fx`). This is what your students will type and what the evaluation function sees.
+*    **Alternatives:** A list of other codes you want to accept for the same symbol (e.g., `f_x`, `f(x)`, `f`). This allows you to anticipate different ways students might type the same thing.
+*    **Visibility:** A `TRUE`/`FALSE` toggle. If "Display input symbols" is enabled in the Input tab, this setting determines whether a specific symbol is shown to the student. This allows you to show students common symbols while still accepting less common or alternative ones in the background.
+
+![example](screenshots/input_symbols.png)
+![example](screenshots/input_symbols_preview.png)
+
+Tolerances can also be added. These will apply to the numerical parts of the answer (e.g. the $10$ in $10x$).
+
+This is done using the `atol` and `rtol` fields under the Evaluation Function Parameters section.
+
 ## Component Parameters
 
 ### `post_response_text` (optional)
@@ -22,11 +39,11 @@ Text block to be displayed to the left of the input field. Markdown and LaTeX ar
 
 Text block to be displayed to the left of the input field. Markdown and LaTeX are allowed following the usual syntax.
 
-### Allow Handwrite (Experimental)
+### Enable Handwriting Input
 
-Enables a handwriting canvas in the browser, which allows a student can use to draw their expression, rather than type using Sympy's syntax.
+Enables a handwriting canvas in the browser, which allows a student to draw their expression, rather than type using Sympy's syntax.
 
-### Photo (Experimental)
+### Enable Photo Upload
 
 Allows a student to upload their expression as an image, as an alternative to handwriting if the student isn't using a phone or tablet.