btrace, doc: Clarify record function-call-history documentation.

btrace, doc: Clarify record function-call-history documentation.

The documentation for 'record function-call-history' mentions lines instead
of functions when talking about the number of functions printed, as currently
there is only one line printed per function.  Yet the code actually handles
this on function granularity not on line basis.  Future patches will
extend the number of lines printed per function.  This also makes it
consistent with the 'record instruction-history' command, where multiple
lines can be printed per instruction.

gdb/doc/ChangeLog:
2021-06-16  Felix Willgerodt  <felix.willgerodt@intel.com>

	* gdb.texinfo (Process Record and Replay): Stop mentioning lines
	for "record function-call-history" and
	"set record function-call-history-size".