update docs to mention bug in loading sketches from sig/sig.gz files in pathlists

[ctb]

ref #430, #445 - #430 did a lot of things, but it did not fix this bug :)

[ctb]

as of v0.9.9 (next release), the branchwater plugin will include sourmash v0.17.0 which per sourmash-bio/sourmash#3333 panics with a mildly informative error message when this situation is encountered. So I think it will be OK to leave it out of the documentation for now, and just make sure that the error message is findable in the issue tracker.

The error message raised here is:

not implemented: only one Signature currently allowed when using 'load_sig'

I'll create a new issue that lays out the situation more nicely.

[ctb]

see #501