Fix a misleading phrase in the LangRef

[hfinkel]

I agree, this is better. Does this working make sense for vector GEPs?

[sanjoy]

• account for vector geps

[hfinkel]

LGTM

llvm/trunk/docs/LangRef.rst

-  An integer constant other than zero or a pointer value returned from
    a function not defined within LLVM may be associated with address
    ranges allocated through mechanisms other than those provided by
    LLVM. Such ranges shall not overlap with any ranges of addresses
    allocated by mechanisms provided by LLVM.
 
 A pointer value is *based* on another pointer value according to the
 following rules:
 
--  A pointer value formed from a ``getelementptr`` operation is *based*
-   on the second value operand of the ``getelementptr``.
+-  A pointer value formed from a scalar ``getelementptr`` operation is *based* on
+   the pointer-typed operand of the ``getelementptr``.
+-  The pointer in lane *l* of the result of a vector ``getelementptr`` operation
+   is *based* on the pointer in lane *l* of the vector-of-pointers-typed operand
+   of the ``getelementptr``.
 -  The result value of a ``bitcast`` is *based* on the operand of the
    ``bitcast``.
 -  A pointer value formed by an ``inttoptr`` is *based* on all pointer
    values that contribute (directly or indirectly) to the computation of
    the pointer's value.
 -  The "*based* on" relationship is transitive.
 
 Note that this definition of *"based"* is intentionally similar to the