Improve documention of essential NULL values to terminate roxygen2 blocks

[mpadge]

Thanks to @helske via https://github.com/s3alfisc/fwildclusterboot/issues/111

These NULLs are critical in defining roxygen2 "blocks". Without them roxygen2 just passes straight over, and so doesn't feed anything at all to srr, leading to standards not being identified. Can't really catch the absence of null here, because roxygen2 doesn't catch those, but can definitely improve docs to emphasise the critical importance of terminal NULLs.

[mpadge]

@s3alfisc Sorry about the confusion in documenting your package. If you have a chance, it'd be helpful if you could look at the commit linked above, and let us know whether that is likely to have been sufficient to have avoided confusion? Alternatively, feel free to suggest other places where we might still improve documentation of the importance of terminal NULLs in all blocks. Thanks!

[s3alfisc]

Yes, it's very clear! =)