Docs (discuss feedback): fix incorrect statement about upsert blocks allowing one query and one mutation

[aaroncarey]

Fixes the incorrect statement in the PR title, per DGRAPH-2777 and https://discuss.dgraph.io/t/f-strings-vs-variables/11606/3

• Some rewording in this section
• Clarification about what upsert blocks are and how to use them
• Updated a deprecated URL pointing to upsert block docs

On-hold pending resolution of TeamCity build errors.

---

This change is 

[CLAassistant]

All committers have signed the CLA.

[anand-chandrashekar]

Hi @aaroncarey , Let's use the same language used in the upsert block documentation. IMO, it's unambiguous.

"The upsert block allows performing queries and mutations in a single request. The upsert block contains one query block and one or more than one mutation blocks. Variables defined in the query block can be used in the mutation blocks using the uid and val function."

[anand-chandrashekar]

Minor alteration:
"The upsert block allows performing queries and mutations in a single request. The upsert block contains one query block and one or more than one mutation blocks. Variables defined in the query block can be used in the mutation blocks using the uid and val function available in DQL."

[bucanero]

Reviewable status: 0 of 1 files reviewed, all discussions resolved (waiting on @anand-chandrashekar and @bucanero)

[aaroncarey]

Thanks for the feedback, Anand! And for the review, Damian! I incorporated your feedback with minor rewording into the latest commit.

Reviewable status: 0 of 1 files reviewed, all discussions resolved (waiting on @anand-chandrashekar)

[aaroncarey]

On hold pending resolution of build integration failures.

[aaroncarey]

Superseded by #162