btrace: add instruction-history /s and fix documentation

btrace: add instruction-history /s and fix documentation

Add support for the /s modifier of the "record instruction-history" command.  It
behaves exactly like /m and prints disassembled instructions in the order in
which they were recorded with interleaved sources.  We accept /s in addition
to /m to align with the "disassemble" command.

The "record instruction-history" modifiers were not documented.  Document
all of them.

gdb/
	* record.c (get_insn_history_modifiers): Set DISASSEMBLY_SOURCE
	instead of DISASSEMBLY_SOURCE_DEPRECATED.  Also accept /s.
	(_initialize_record): Document the /s modifier.
	* NEWS: Announce record instruction-history's new /s modifier.

doc/
	* gdb.texinfo (Process Record and Replay): Document "record
	instruction-history" modifiers.