fix: Update documentation for conflict to include table alias

[jsk95]

Problem

The current documentation for where after onConflict + merge does not seem correct.
Following the example, we receive the error column reference "x" is ambiguous.

Root cause

This is because in the where condition of a onConflict + merge, we have access to both the table and excluded aliases so specifying a column without being explicit results in that error.
https://www.postgresql.org/docs/current/sql-insert.html
https://dba.stackexchange.com/a/224167

We can actually see the table name being directly referenced in the test case that this scenario was created for https://github.com/knex/knex/pull/4148/files#diff-57a163701cfad49b7c6cbdfb102667277345ba8de7e575f6966269b0c0d8d3a0R1557

Solution

We could update with a custom where condition to actually use table name
https://github.com/knex/knex/blame/4ca3dd5bc28e0665c5bed55026fac2ec45489d81/lib/dialects/postgres/query/pg-querycompiler.js#L37

but implicitly choosing one or the other feels dangerous.

So this proposal is just to fix the documentation that shows the feature to work.

[coveralls]

coverage: 92.843%. remained the same
when pulling 792a4ea on jsk95:patch-1
into 9659a20 on knex:master.