[Docs] Update migration recommendations with support for ALTER TABLE...ADD PRIMARY KEY

[mrajcevic01]

Description

ALTER TABLE...ADD PRIMARY KEY is now supported in an offline fashion via #1104.

We need to update our messaging in our YSQL Migration docs that it is now supported. We should add that although this is now supported it requires a disk re-write and may be resource intensive, hence we still recommending setting the PK inline. This may not be a big deal for clusters with no data but it is still done in an offline manner so there should be no active reads/writes.