Add info about changes introduced by PR #4531

[animesh2049]

Now we allow filtering based on non-idexed predicate for comparision functions.

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This change is 

[sleto-it]

Hi, one comment from me. In general, it would be extremely useful, when something changes or is a totally new feature, that we document the version when the change happened or when the feature was introduced

Examples for new features:

a) [...]. This option has been introduced in vX.Y
b) Starting from vX.Y, it is now possible to [...]

Examples for changes:

a) Beginning with vX.Y, [...]. Previously {quick description of old behavior)

This will help a lot both users and support team

Examples / reference (from MySQL and MongoDB):

1. https://dev.mysql.com/doc/refman/8.0/en/with.html

2. https://dev.mysql.com/doc/refman/8.0/en/comparison-operators.html

3. https://docs.mongodb.com/manual/reference/method/db.getLastErrorObj/

Thanks,

[campoy]

Reviewable status: 0 of 1 files reviewed, 1 unresolved discussion (waiting on @animesh2049, @campoy, @danielmai, and @MichaelJCompton)

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wiki/content/query-language/index.md, line 217 at r1 (raw file):

Comparison functions in the query root (aka func:) can only be applied on [indexed predicates]({{< relref "#indexing">}}).
Since v1.2, comparison functions can now be used on @filter directives even on predicates that have not been indexed.
Filtering on non-indexed predicates can be slow for large datasets, as they require iterating over all of the possible values at the level where the filter is being used.

And add a bunch fo links to "query root", "filter", etc

Not sure this needs to be inside of a notice, though ... maybe on the notice just add "Support for filters on non-indexed predicates was added with Dgraph v1.2.0" and let the explanation above right after.

[campoy]

Reviewed 1 of 1 files at r1.
Reviewable status: all files reviewed, 1 unresolved discussion (waiting on @animesh2049, @danielmai, and @MichaelJCompton)

[campoy]

Reviewed 1 of 1 files at r2.
Reviewable status: all files reviewed, 1 unresolved discussion (waiting on @animesh2049, @danielmai, and @MichaelJCompton)