docs: --singleton-table DDL strategy flag (#1881)

* docs: --singleton-table DDL strategy flag

Signed-off-by: Shlomi Noach <2607934+shlomi-noach@users.noreply.github.com>

* Update content/en/docs/22.0/user-guides/schema-changes/ddl-strategy-flags.md

Co-authored-by: Matt Lord <mattalord@gmail.com>
Signed-off-by: Shlomi Noach <2607934+shlomi-noach@users.noreply.github.com>

* pending vs running

Signed-off-by: Shlomi Noach <2607934+shlomi-noach@users.noreply.github.com>

---------

Signed-off-by: Shlomi Noach <2607934+shlomi-noach@users.noreply.github.com>
Co-authored-by: Matt Lord <mattalord@gmail.com>

Author: shlomi-noach

content/en/docs/22.0/user-guides/schema-changes/ddl-strategy-flags.md

@@ -47,6 +47,8 @@ Vitess respects the following flags. They can be combined unless specifically in
 
   It does not make sense to combine `--singleton` and `--singleton-context`.
 
+- `--singleton-table`: reject a new submission for a table or view which already has a pending migration.
+
 ## Pass-through flags
 
 Flags unrecognized by Vitess are passed on to the underlying schema change tools. For example, a `gh-ost` migration can run with: