Add compatibility note for clang-format and fix a small issue in .clang-format

[SunBlack]

I tried to make clang-format 10 compatible to 7-9, but I didn't found a way to do so. So basically I compared result of clang-format-8 -dump-config > .clang-format8 with clang-format-10 -dump-config > .clang-format10. As no default value of an option was changed and we cannot use newer options, as defining them breaks clang-tidy for old version (you can't tell clang-tidy to ignore unknown options) and even no new option sounds like it is the reason for the slightly other formatting, I added a note that PCL is only compatible to clang-tidy 7-9 and not 10.

Example of difference between clang-tidy 8 and 10:

[taketwo]

👍 for the fix in .clang-format

Regarding the formatting change, it seems to me that the old clang-format has a bug that was fixed in the newer version. This is the current formatting:

pcl/octree/include/pcl/octree/octree2buf_base.h

Lines 150 to 158 in 10230bc

	/** \brief Get reference to container */
	ContainerT& operator*() { return container_; }
	/** \brief Get const reference to container */
	const ContainerT&
	getContainer() const
	{
	return container_;
	}

Apparently, when formatting operator members, old clang-format does not respect our rules about one-liners and template declaration breaking.

I think the way forward is to switch to the newest version (with correct behavior) on CI and add a note that old versions make mistakes.

[SunBlack]

Switching to clang-format 10 also also an option, as the result looks better for me

Addition: The difference between clang-format 8 and 10 basing on your .clang-format file:

[kunaltyagi]

I'd vote for switching to 10, and reporting that in certain conditions, the output may differ (which has always been the case)

[SunBlack]

Closing in favor of #3903.