[NaClDocs] Add documentation for the PNaCl fabs intrinsic.

native_client_sdk/src/doc/reference/pnacl-bitcode-abi.rst

 ==============================
 PNaCl Bitcode Reference Manual
 ==============================
 
 .. contents::
    :local:
    :backlinks: none
    :depth: 3
 
 Introduction
@@ skipping 412 matching lines @@
 
 * ``llvm.bswap``
 
   The overloaded ``llvm.bswap`` intrinsic is only supported with the following
   argument types: i16, i32, i64 (the types supported by C-style GCC builtins).
 
 * ``llvm.ctlz``
 * ``llvm.cttz``
 * ``llvm.ctpop``
 
-  The overloaded llvm.ctlz, llvm.cttz, and llvm.ctpop intrinsics are only
-  supported with the i32 and i64 argument types (the types supported by
-  C-style GCC builtins).
+  The overloaded ``llvm.ctlz``, ``llvm.cttz``, and ``llvm.ctpop`` intrinsics
+  are only supported with the i32 and i64 argument types (the types
+  supported by C-style GCC builtins).
+
+* ``llvm.fabs``
+
+  The overloaded ``llvm.fabs`` intrinsic is supported for float, double and
+  ``<4 x float>`` argument types. It returns the absolute value of
+  the argument. Some notable points: it returns ``+0.0`` when given ``-0.0``,
+  ``+inf`` when given ``-inf``, and a positive ``NaN`` when given any
+  signed ``NaN``.
+
+  NOTE: This intrinsic was introduced in the pepper_42 SDK.
 
 * ``llvm.sqrt``
 
   The overloaded ``llvm.sqrt`` intrinsic is only supported for float
   and double arguments types. This has the same semantics as the libc
-  sqrt function, returning NaN for values less than -0.0. However, this
-  does not set ``errno`` when the result is NaN (see the
+  sqrt function, returning ``NaN`` for values less than ``-0.0``.
+  However, this does not set ``errno`` when the result is NaN (see the
   :ref:`instructions and errno <ir_and_errno>` section).
 
 * ``llvm.stacksave``
 * ``llvm.stackrestore``
 
   These intrinsics are used to implement language features like scoped automatic
   variable sized arrays in C99. ``llvm.stacksave`` returns a value that
   represents the current state of the stack. This value may only be used as the
   argument to ``llvm.stackrestore``, which restores the stack to the given
   state.
@@ skipping 114 matching lines @@
 
     declare i1 @llvm.nacl.atomic.is.lock.free(i32 <byte_size>, i8* <address>)
 
 The ``llvm.nacl.atomic.is.lock.free`` intrinsic is designed to
 determine at translation time whether atomic operations of a certain
 ``byte_size`` (a compile-time constant), at a particular ``address``,
 are lock-free or not. This reflects the C11 ``atomic_is_lock_free``
 function from header ``<stdatomic.h>`` and the C++11 ``is_lock_free``
 member function in header ``<atomic>``. It can be used through the
 ``__nacl_atomic_is_lock_free`` builtin.