Add documentation for the attribute "no-jump-tables"

[sgundapa]

Hans, I am not sure if this is the correct place and correct way to add the documentation. Let me know

[hans]

Yes, I think this is the right place. Thanks.

[sgundapa]

This patch is committed in r309445

docs/LangRef.rst

 ``minsize``
     This attribute suggests that optimization passes and code generator
     passes make choices that keep the code size of this function as small
     as possible and perform optimizations that may sacrifice runtime
     performance in order to minimize the size of the generated code.
 ``naked``
     This attribute disables prologue / epilogue emission for the
     function. This can have very system-specific consequences.
+``no-jump-tables``
+    When this attribute is set to true, the jump tables and lookup tables that
+    can be generated from a switch case lowering are disabled.
 ``nobuiltin``
     This indicates that the callee function at a call site is not recognized as
     a built-in function. LLVM will retain the original call and not replace it
     with equivalent code based on the semantics of the built-in function, unless
     the call site uses the ``builtin`` attribute. This is valid at call sites
     and on function declarations and definitions.
 ``noduplicate``
     This attribute indicates that calls to the function cannot be