Clarify in join doc that fields can be copied from the right side without assignment

[philrz]

What's changing

A small clarification is being made to the join operator doc.

Why

A community user recently spotted this omission.

Details

In the user's own words from the community Slack thread:

for joins - I feel like the syntax here in the docs could somehow also illustrate that just the name of a field from the right-side can be included without assignment:
now:

<left-input>
| [anti|inner|left|right] join (
 <right-input>
) on <left-key>=<right-key> [<field>:=<right-expr>, ...]

off-the-cuff proposal:

<left-input>
| [anti|inner|left|right] join (
 <right-input>
) on <left-key>=<right-key> [<field>:=<right-expr>, <right-field>, ...]

At first I toyed with updating it to [<field>:=<right-expr>|<right-field>, ...], but that may give the impression that only an explicit right-side field name would be supported, but expressions such as function calls cook up field names on the fly, e.g., this change from the Inner Join example in the join tutorial:

$ zq -z '
file fruit.ndjson | sort flavor
| inner join (
  file people.ndjson | sort likes
) on flavor=likes upper(name)'
{name:"figs",color:"brown",flavor:"plain",upper:"JESSIE"}
{name:"banana",color:"yellow",flavor:"sweet",upper:"QUINN"}
{name:"strawberry",color:"red",flavor:"sweet",upper:"QUINN"}
{name:"dates",color:"brown",flavor:"sweet",note:"in season",upper:"QUINN"}
{name:"apple",color:"red",flavor:"tart",upper:"MORGAN"}
{name:"apple",color:"red",flavor:"tart",upper:"CHRIS"}

So instead I've taken the approach of making the assignment portion appear optional, knowing that errors will further communicate the limitations if the user tries something unworkable, e.g.,

$ zq -z '
file fruit.ndjson | sort flavor
| inner join (
  file people.ndjson | sort likes
) on flavor=likes name+"hi"
'
zq: cannot infer field from expression at line 5, column 19:
) on flavor=likes name+"hi"
                  ~~~~~~~~~
illegal left-hand side of assignment at line 5, column 19:
) on flavor=likes name+"hi"
                  ~~~~~~~~~