docs(diff): update --filter CLI help text

Update the short help text for the --filter parameter to document
all valid filter options (added, modified, renamed, dropped) and
mention that 'removed' is accepted as an alias for 'dropped'.

Refs: #1430

Author: codeaucafe

go/cmd/dolt/cli/arg_parser_helpers.go

@@ -311,7 +311,7 @@ func CreateDiffArgParser(isTableFunction bool) *argparser.ArgParser {
 		ap.SupportsString(FormatFlag, "r", "result output format", "How to format diff output. Valid values are tabular, sql, json. Defaults to tabular.")
 		ap.SupportsString(WhereParam, "", "column", "filters columns based on values in the diff.  See {{.EmphasisLeft}}dolt diff --help{{.EmphasisRight}} for details.")
 		ap.SupportsInt(LimitParam, "", "record_count", "limits to the first N diffs.")
-		ap.SupportsString(FilterParam, "", "diff_type", "filters results based on the type of modification (added, modified, removed).")
+		ap.SupportsString(FilterParam, "", "diff_type", "filters results based on the type of change (added, modified, renamed, dropped). 'removed' is accepted as an alias for 'dropped'.")
 		ap.SupportsFlag(StagedFlag, "", "Show only the staged data changes.")
 		ap.SupportsFlag(CachedFlag, "c", "Synonym for --staged")
 		ap.SupportsFlag(MergeBase, "", "Uses merge base of the first commit and second commit (or HEAD if not supplied) as the first commit")