fix: mark abstract-declarator in function parameter-list as optional

Author: Sudrut

src/content/docs/cpp/language/function.mdx

@@ -303,15 +303,17 @@ The parameter list determines the arguments that can be specified when the funct
   <Decl slot="decl">
     <RevisionBlock until="C++11" noborder>
       ```cpp
-      /* decl-specifier-seq */ /* declarator */ = /* initializer */
+      /* decl-specifier-seq */ /* declarator */
+        = /* initializer */
       ```
     </RevisionBlock>
   </Decl>
   <Decl slot="decl">
     <RevisionBlock since="C++11" noborder>
       ```cpp
       /* attr (optional) */
-      /* decl-specifier-seq */ /* declarator */ = /* initializer */
+      /* decl-specifier-seq */ /* declarator */
+        = /* initializer */
       ```
     </RevisionBlock>
   </Decl>
@@ -327,15 +329,15 @@ The parameter list determines the arguments that can be specified when the funct
   <Decl slot="decl">
     <RevisionBlock until="C++11" noborder>
       ```cpp
-      /* decl-specifier-seq */ /* abstract-declarator */
+      /* decl-specifier-seq */ /* abstract-declarator (optional) */
       ```
     </RevisionBlock>
   </Decl>
   <Decl slot="decl">
     <RevisionBlock since="C++11" noborder>
       ```cpp
       /* attr (optional) */
-      /* decl-specifier-seq */ /* abstract-declarator */
+      /* decl-specifier-seq */ /* abstract-declarator (optional) */
       ```
     </RevisionBlock>
   </Decl>
@@ -352,7 +354,7 @@ The parameter list determines the arguments that can be specified when the funct
     <RevisionBlock since="C++23" noborder>
       ```cpp
       /* attr (optional) */
-      this /* decl-specifier-seq */ /* abstract-declarator */
+      this /* decl-specifier-seq */ /* abstract-declarator (optional) */
       ```
     </RevisionBlock>
   </Decl>
@@ -364,15 +366,17 @@ The parameter list determines the arguments that can be specified when the funct
   <Decl slot="decl">
     <RevisionBlock until="C++11" noborder>
       ```cpp
-      /* decl-specifier-seq */ /* abstract-declarator */ = /* initializer */
+      /* decl-specifier-seq */ /* abstract-declarator (optional) */
+        = /* initializer */
       ```
     </RevisionBlock>
   </Decl>
   <Decl slot="decl">
     <RevisionBlock since="C++11" noborder>
       ```cpp
       /* attr (optional) */
-      /* decl-specifier-seq */ /* abstract-declarator */ = /* initializer */
+      /* decl-specifier-seq */ /* abstract-declarator (optional) */
+        = /* initializer */
       ```
     </RevisionBlock>
   </Decl>