From perlre, identical in both Strawberry 126.96.36.199 local docs and 5.16.2 on-line docs (emphases added):
\K  Keep the stuff left of the \K, don't include it in $&
(Note  simply refers to the discussion in the "Look-Around Assertions" section.)
From "Look-Around Assertions":
There is a special form of this construct, called "\K",
which causes the regex engine to "keep" everything it had
matched prior to the "\K" and not include it in $&.
Hmmm... I suppose when I read these sections heretofore, my attention was drawn to the "don't include..." clauses and I more or less ignored the "left of" and "prior to" statements. My naive association of "keep" is with inclusion: in this specific instance, everything to the right of \K is (or could be) included in $&. The docs seem to be using "keep" in the sense of "keep back", i.e., "to exclude": everything to the left of \K is "kept back" from inclusion in $&. Perhaps they are discussing the behavior \K in the context of substitution: everything left of the \K is not included in $& and so cannot be zapped by substitution: it is "kept safe" from substitution. But \K is equally valid and useful in both m// matching and s/// substitution, so...
I can't say I would agree with the perhaps subtle interpretation given to "keep" in the documentation, but it is fully clarified by the "don't include" language and by the examples, so I doubt this qualifies as a doc-bug.
Update: LanX: Almost as confusing like using single line and multi line modifier at the same time... ++That!