Patching documentation of ALTER TABLE re column type changes on binary-coercible fields

Hi, first patch here and first post to pgsql-hackers. Here goes.

Enclosed please find a patch to tweak the documentation of the ALTER TABLE
page. I believe this patch is ready to be applied to master and backported
all the way to 9.2.

On the ALTER TABLE page, it currently notes that if you change the type of
a column, even to a binary coercible type:

> any indexes on the affected columns must still be rebuilt.

It appears this hasn't been true for about eight years, since 367bc426a.

Here's the discussion of the topic from earlier today and yesterday:

https://www.postgresql.org/message-id/flat/CAMp9%3DExXtH0NeF%2BLTsNrew_oXycAJTNVKbRYnqgoEAT01t%3D67A%40mail.gmail.com

I haven't run tests, but I presume they'll be unaffected by a documentation
change.

I've made an effort to follow the example of other people's patches I
looked at, but I haven't contributed here before. Happy to take another
stab at this if this doesn't hit the mark — though I hope it does. I love
and appreciate Postgresql and hope that I can do my little part to make it
better.

For the moment, I haven't added this to commitfest. I don't know what it
is, but I suspect this is small enough somebody will just pick it up.

Mike