Address Copilot review: comment placement, %expect docs, test wording

1. Move "Helper function to create an ExplainStmt node" comment from
   above make_exists_pattern_sublink() to above make_explain_stmt()
   where it belongs.

2. Add block comment documenting the %expect/%expect-rr conflict
   budget: 7 S/R from path vs arithmetic on - and <, 3 R/R from
   expr_var vs var_name_opt on ) } =.

3. Clarify test comment: "Regular expressions" -> "Regular (non-pattern)
   expressions" to avoid confusion with regex.

Regression test: pattern_expression OK.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: gregfelice

regress/expected/pattern_expression.out

@@ -193,7 +193,7 @@ $$) AS (result agtype);
 (1 row)
 
 --
--- Regular expressions still work (no regression)
+-- Regular (non-pattern) expressions still work (no regression)
 --
 SELECT * FROM cypher('pattern_expr', $$
     RETURN (1 + 2)

regress/sql/pattern_expression.sql

@@ -141,7 +141,7 @@ SELECT * FROM cypher('pattern_expr', $$
 $$) AS (result agtype);
 
 --
--- Regular expressions still work (no regression)
+-- Regular (non-pattern) expressions still work (no regression)
 --
 SELECT * FROM cypher('pattern_expr', $$
     RETURN (1 + 2)

src/backend/parser/cypher_gram.y

@@ -58,6 +58,19 @@
  * where both paths succeed (bare (a) prefers the expression interpretation).
  */
 %glr-parser
+/*
+ * Conflict budget for the GLR parser.  Update these counts if grammar
+ * rules change.
+ *
+ *   %expect 7 (shift/reduce) -- All arise from the ambiguity between
+ *     path extension ('-' '[' ... ']' '-' '>') and arithmetic operators
+ *     on '-' and '<'.  GLR forks at these points and discards the
+ *     failing alternative.
+ *
+ *   %expect-rr 3 (reduce/reduce) -- From the overlap between expr_var
+ *     and var_name_opt on ')' / '}' / '='.  Resolved by %dprec
+ *     annotations that prefer the expression interpretation.
+ */
 %expect 7
 %expect-rr 3
 
@@ -3328,7 +3341,6 @@ static Node *build_list_comprehension_node(Node *var, Node *expr,
     return (Node *) node_to_agtype((Node *)sub, "agtype[]", location);
 }
 
-/* Helper function to create an ExplainStmt node */
 /*
  * Wrap a graph pattern in an EXISTS SubLink.  Used by both
  * EXISTS(pattern) syntax and bare pattern expressions in WHERE.
@@ -3353,6 +3365,7 @@ static Node *make_exists_pattern_sublink(Node *pattern, int location)
     return (Node *)node_to_agtype((Node *)n, "boolean", location);
 }
 
+/* Helper function to create an ExplainStmt node */
 static ExplainStmt *make_explain_stmt(List *options)
 {
     ExplainStmt *estmt = makeNode(ExplainStmt);