xz: Add a section to man page for robot mode --filters-help.

src/xz/xz.1

@@ -940,7 +940,7 @@ A special value of
 may be used as the last size to indicate that
 the rest of the file should be encoded as a single block.
 .IP ""
-An alternative custom filter chain for each block can be
+An alternative filter chain for each block can be
 specified in combination with the
 .BI \-\-filters1= filters
 \&...\&
@@ -1343,7 +1343,7 @@ and
 .BI \-\-filters1= filters
 \&...\&
 .BI \-\-filters9= filters
-allows specifying an entire filter chain in one option using the
+allow specifying an entire filter chain in one option using the
 liblzma filter string syntax.
 Alternatively, a filter chain can be specified by using one or more
 individual filter options in the order they are wanted in the filter chain.
@@ -2050,6 +2050,7 @@ Currently
 .B \-\-robot
 is supported only together with
 .BR \-\-list ,
+.BR \-\-filters\-help ,
 .BR \-\-info\-memory ,
 and
 .BR \-\-version .
@@ -2340,6 +2341,33 @@ Future versions may add new line types and
 new columns can be added to the existing line types,
 but the existing columns won't be changed.
 .
+.SS "Filters help"
+.B "xz \-\-robot \-\-filters-help"
+prints the supported filters in the following format:
+.PP
+\fIfilter\fB:\fIoption\fB=<\fIvalue\fB>,\fIoption\fB=<\fIvalue\fB>\fR...
+.TP
+.I filter
+Name of the filter
+.TP
+.I option
+Name of a filter specific option
+.TP
+.I value
+Numeric
+.I value
+ranges appear as
+\fB<\fImin\fB\-\fImax\fB>\fR.
+String
+.I value
+choices are shown within
+.B "< >"
+and separated by a
+.B |
+character.
+.PP
+Each filter is printed on its own line.
+.
 .SS "Memory limit information"
 .B "xz \-\-robot \-\-info\-memory"
 prints a single line with three tab-separated columns: