Fix rendering of default field values in reference docs. (pantsbuild#…

…13891)

Readme renders some markdown even inside <pre>/<code> tags,
including, as it turns out, * and _. So we turn them into
the corresponding HTML entities.

[ci skip-rust]

[ci skip-build-wheels]

build-support/bin/generate_docs.py

@@ -332,7 +332,9 @@ def munge_option(option_data):
             else:
                 # It should already be a string, but might as well be safe.
                 default_str = to_help_str(default_help_repr)
-            escaped_default_str = html.escape(default_str, quote=False)
+            escaped_default_str = (
+                html.escape(default_str, quote=False).replace("*", "*").replace("_", "_")
+            )
             if "\n" in default_str:
                 option_data["marked_up_default"] = f"<pre>{escaped_default_str}</pre>"
             else:
@@ -354,7 +356,11 @@ def process_targets_input(cls, help_info: dict[str, Any]) -> dict[str, dict[str,
         for target in target_info.values():
             for field in target["fields"]:
                 # Combine the `default` and `required` properties.
-                default_str = html.escape(str(field["default"]))
+                default_str = (
+                    html.escape(str(field["default"]))
+                    .replace("*", "&ast;")
+                    .replace("_", "&lowbar;")
+                )
                 field["default_or_required"] = (
                     "required" if field["required"] else f"default: <code>{default_str}</code>"
                 )