Expected NA values and therefore, warnings when plotting

[valentinitnelav]

Make it clear via a note that is expected to see lots of warning messages when plotting because some bootstrap values are NA (during the sampling procedure, some interactions are rare , so at small sample sizes, they are not sampled, they only appear later).

This happens particularly for species level indices. It can also happen for network levels indices when maybe the start sample size (first sampled interactions) is too small and metrics cannot be computed. Then because of how try is set, an NA is assigned instead of failing (which should stay like this).