Document filter and modification syntax

cli/src/argparse/modification.rs

@@ -1,8 +1,10 @@
 use super::args::{any, arg_matching, minus_tag, plus_tag};
 use super::ArgList;
+use crate::usage;
 use nom::{branch::alt, combinator::*, multi::fold_many0, IResult};
 use std::collections::HashSet;
 use taskchampion::Status;
+use textwrap::dedent;
 
 #[derive(Debug, PartialEq, Clone)]
 pub enum DescriptionMod {
@@ -107,6 +109,34 @@ impl Modification {
         }
         map_res(arg_matching(minus_tag), to_modarg)(input)
     }
+
+    pub(super) fn get_usage(u: &mut usage::Usage) {
+        u.modifications.push(usage::Modification {
+            syntax: "DESCRIPTION".to_owned(),
+            summary: "Set description".to_owned(),
+            description: dedent(
+                "
+                Set the task description.  Multiple arguments are combined into a single
+                space-separated description.",
+            ),
+        });
+        u.modifications.push(usage::Modification {
+            syntax: "+TAG".to_owned(),
+            summary: "Tag task".to_owned(),
+            description: dedent(
+                "
+                Add the given tag to the task.",
+            ),
+        });
+        u.modifications.push(usage::Modification {
+            syntax: "-TAG".to_owned(),
+            summary: "Un-tag task".to_owned(),
+            description: dedent(
+                "
+                Remove the given tag from the task.",
+            ),
+        });
+    }
 }
 
 #[cfg(test)]