doc: Bring mention of unique index forced transaction wait behavior outside of the internal section

Hi.

The fact that one transaction will wait on another if they are trying to
claim the same unique value is presently relegated to a subchapter of the
documentation where the typical reader will not even understand (rightly
so) the main chapter's title. This has prompted a number of questions
being sent to the mailing list over the years about a topic we do cover in
some detail in the documentation, most recently here:

https://www.postgresql.org/message-id/CAJQY8UosNct0m0xbD7gkWGs02c0SOZN1DET-Q94jjpV1LrC2SQ@mail.gmail.com

Attached is a proposal for incorporating some high-level detail within the
MVCC Chapter, where readers are already looking to learn about how
transactions interact with each other and are "isolated" (or not, in this
case) from each other.

I'm neither married to the verbiage nor location but it seems better than
nothing and a modest improvement for not much effort. It's basically a
glorified cross-reference. I didn't dislike directing the reader to the
internals section enough to try and establish a better location for
the main content. It just needs better navigation to it from places the
reader is expected to peruse.

David J.