doc: RISC-V: Document that misaligned accesses are supported

The RISC-V ISA manual used to mandate that misaligned accesses were
supported in user mode, but that requirement was removed in 2018 via
riscv-isa-manual commit 61cadb9 ("Provide new description of misaligned
load/store behavior compatible with privileged architecture.").  Since
the Linux uABI was already frozen at that point it's just been demoted
to part of the uABI, but that was never written down.

Link: https://lore.kernel.org/r/20220728210715.17214-1-palmer@rivosinc.com
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>

Documentation/riscv/index.rst

@@ -8,6 +8,7 @@ RISC-V architecture
     boot-image-header
     vm-layout
     patch-acceptance
+    uabi
 
     features
 

Documentation/riscv/uabi.rst

@@ -0,0 +1,6 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+RISC-V Linux User ABI
+=====================
+
+Misaligned accesses are supported in userspace, but they may perform poorly.