Document substitution variables.

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+Variable Names [substvars-name]
+===============================
+
+Variable names may only consist of uppercase and lowercase Latin letters,
+digits, and hyphens.  Names must be at least one character long.  (A regular
+expression for this is `[A-Za-z0-9-]+`.)
+
+
+Variable Substitutions [substvars-exp]
+======================================
+
+Variable substitutions are performed on certain binary control field values, as
+explained in $[sectlink][fields-bin].
+
+The format for variable substitutions is as follows:
+
+    ${<var-name>}
+
+`<var-name>` is the name of the variable.