[spec/grammar] Add key node diagrams (#4272)

* [spec/grammar] Add key node diagrams

Add comment about avoiding GLINK2 macro here.
Mention AggregateDeclaration.
Mention control-flow statements.
Describe key *Expression* nodes.

* More Expression docs

*AssignExpression* and *PostfixExpression* each have at least one kind of
*Expression*.
Every *Expression* has a *Type*.

* *AggregateDeclaration* has *DeclDef* members

Author: ntrel

images/declaration.dot

@@ -0,0 +1,16 @@
+/*
+Source file for declaration.svg and declaration.eps. The
+commands to run (assuming graphviz is installed) are:
+
+cd /path/to/dlang.org/images
+dot -Tsvg declaration.dot >declaration.svg
+dot -Teps declaration.dot >declaration.eps
+
+These commands are not part of the makefile because the files are fairly stable
+and so as to not make the build depending on graphviz being installed.
+*/
+
+digraph "Declaration" {
+    rankdir="LR";
+    "Declaration" -> {"VarDeclarations", "FuncDeclaration", "AggregateDeclaration"};
+}

images/declaration.eps

@@ -0,0 +1,17 @@
+/*
+Source file for expression.svg and expression.eps. The
+commands to run (assuming graphviz is installed) are:
+
+cd /path/to/dlang.org/images
+dot -Tsvg expression.dot >expression.svg
+dot -Teps expression.dot >expression.eps
+
+These commands are not part of the makefile because the files are fairly stable
+and so as to not make the build depending on graphviz being installed.
+*/
+
+digraph "Expression" {
+    rankdir="LR";
+    "Expression" -> {"PrimaryExpression", "AssignExpression", "PostfixExpression"}
+    "PrimaryExpression" -> {"Identifier", "LiteralExpression"};
+}