Clarify the 'interrupt' names in Attribute Docs

[erichkeane]

Summary

All 4 of the 'interrupt' headers were automatically named
'interrupt'. This patch gives them unique names.

[aaron.ballman]

LGTM, thank you!

include/clang/Basic/AttrDocs.td

.. code-block:: c++
   case 77:  // warning: unannotated fall-through
     r();
   }
   }];
 }
 
 def ARMInterruptDocs : Documentation {
   let Category = DocCatFunction;
+  let Heading = "interrupt (ARM)";
   let Content = [{
 Clang supports the GNU style ``__attribute__((interrupt("TYPE")))`` attribute on
 ARM targets. This attribute may be attached to a function definition and
 instructs the backend to generate appropriate function entry/exit code so that
 it can be used directly as an interrupt service routine.
 
 The parameter passed to the interrupt attribute is optional, but if
 provided it must be a string literal with one of the following values: "IRQ",
- If the CPU is not M-class, the return instruction is changed to one of the
   handler, where the offset from "lr" to the preferred return address depends on
   the execution state of the code which generated the exception. In this case
   a sequence equivalent to "movs pc, lr" will be used.
   }];
 }
 
 def MipsInterruptDocs : Documentation {
   let Category = DocCatFunction;
+  let Heading = "interrupt (MIPS)";
   let Content = [{
 Clang supports the GNU style ``__attribute__((interrupt("ARGUMENT")))`` attribute on
 MIPS targets. This attribute may be attached to a function definition and instructs
 the backend to generate appropriate function entry/exit code so that it can be used
 directly as an interrupt service routine.
 
 By default, the compiler will produce a function prologue and epilogue suitable for
 an interrupt service routine that handles an External Interrupt Controller (eic)

 
 These attributes take priority over command line switches such
 as ``-mlong-calls`` and ``-mno-long-calls``.
   }];
 }
 
 def AVRInterruptDocs : Documentation {
   let Category = DocCatFunction;
+  let Heading = "interrupt (AVR)";
   let Content = [{
 Clang supports the GNU style ``__attribute__((interrupt))`` attribute on
 AVR targets. This attribute may be attached to a function definition and instructs
 the backend to generate appropriate function entry/exit code so that it can be used
 directly as an interrupt service routine.
 
 On the AVR, the hardware globally disables interrupts when an interrupt is executed.
 The first instruction of an interrupt handler declared with this attribute is a SEI
public:
       }
     };
 
   }];
 }
 
 def AnyX86InterruptDocs : Documentation {
   let Category = DocCatFunction;
+  let Heading = "interrupt (X86)";
   let Content = [{
 Clang supports the GNU style ``__attribute__((interrupt))`` attribute on
 x86/x86-64 targets.The compiler generates function entry and exit sequences
 suitable for use in an interrupt handler when this attribute is present.
 The 'IRET' instruction, instead of the 'RET' instruction, is used to return
 from interrupt or exception handlers.  All registers, except for the EFLAGS
 register which is restored by the 'IRET' instruction, are preserved by the
 compiler.