Docs: Mention ALTER TABLE limits on DDL page

[cweiske]

The ALTER TABLE documentation says nothing about the issue that columns cannot be changed: https://crate.io/docs/reference/sql/ddl.html#alter-table

Please mention that there directly, or at least link directly to https://crate.io/docs/reference/sql/crate-standard-sql.html#alter-table

[chaudum]

hi @cweiske thanks for pointing it out. we will have a look how we can improve the documentation about these limitations.

[mfussenegger]

I think this has been resolved with https://github.com/crate/crate/commit/e916527cfbd9bf571bfde2b1039371207b07620f