Fix the behavior of ${current-pc-arrow} in the default disassembly
format for the not-current-stack-frame.  This was causing
test/functionalities/inferior-assert to fail.

Also document the new additions to the format specifications used
in the disassembly-format changes to formats.html.

<rdar://problem/19102757> 


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@223096 91177308-0d34-0410-b5e6-96231b3b80d8