perf dlfilter: Amend documentation wrt library dependencies

Like all locally-built programs, dlfilters may need to be re-built if
shared libraries they use change. Also there may be unexpected results
if the dfilter uses different versions of the shared libraries that perf
uses.

Note those things in the documentation.

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Link: https //lore.kernel.org/r/20210811101036.17986-5-adrian.hunter@intel.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>