Document how :var introduces code block dependencies.

* doc/org.texi: Document how :var introduces code block dependencies.

doc/org.texi

@@ -13211,6 +13211,15 @@ include anything in the Org mode file that takes a @code{#+NAME:},
 @code{#+BEGIN_EXAMPLE} blocks, other code blocks, and the results of other
 code blocks.
 
+When a reference is made to another code block, the referenced block will be
+evaluated whenever needed, in order to supply its value to the referencing
+block.  If the referenced block is cached (see @ref{cache}), its value will
+be reused if possible, instead of being re-calculated.  If the referring code
+block is cached, its hash value will depend on the value of all the code
+blocks it references.  This system can thus be used to create a system of
+as-needed re-evaluation among code blocks similar to that provided by
+@uref{http://yihui.name/knitr/, knitr} or Sweave.
+
 Argument values can be indexed in a manner similar to arrays (see @ref{var,
 Indexable variable values}).