gdb: fix documentation of gdbarch_displaced_step_copy_insn

gdb: fix documentation of gdbarch_displaced_step_copy_insn

I spotted something that looks wrong in the doc of
gdbarch_displaced_step_copy_insn.

It says that if the function returns NULL, it means that it has emulated
the behavior of the instruction and written the result to REGS.
However, it says below that the function may return NULL to indicate
that the instruction can't be single-stepped out-of-line, in which case
the core steps the instruction in-line.  The two are contradictory.

The right one is the latter, if the function returns NULL, the core
falls back to in-line stepping. I checked all the implementations of
this function and they all agree with this.

gdb/ChangeLog:

	* gdbarch.sh (displaced_step_copy_insn): Update doc.
	* gdbarch.h: Re-generate.

Change-Id: I98163cdd38970cde4c77680e249b10f5d2d5bf9b