cli: Document `--filters` flag of `object search` command

[cthulhu-rider]

Originally posted by @aprasolova

format of object filters in neofs-cli object search --filters ... command is not specified anywhere outside the code. I't not obvious how to set attribute matcher, e.g.:

# works
--filters 'FilePath EQ cat.jpg'

cmp with

# doesn't work
--filters 'FilePath=cat.jpg'

command help doesn't reveal the format too:

$ neofs-cli object search -h
-f, --filters strings       Repeated filter expressions or files with protobuf JSON

i propose to describe format with examples for most wanted cases