head/tail/top error messages should speak of constant expressions

[philrz]

tl;dr

This error message speaks of an expression/integer, but it actually takes a compile-time constant expression that evaluates to a positive integer, which is more limiting.

$ zq 'yield {x:1} | head x'
zq: expression value must be an integer value: error("missing") at line 1, column 20:
yield {x:1} | head x
                   ~

Details

Repro is with Zed commit c1da204.

This topic was surfaced in a recent community Slack thread. Similar error messages show up with head, tail, and top.

$ zq -version
Version: v1.18.0

$ zq 'yield {x:1} | head x'
zq: expression value must be an integer value: error("missing") at line 1, column 20:
yield {x:1} | head x
                   ~

$ zq 'yield {x:1} | tail x'
zq: expression value must be an integer value: error("missing") at line 1, column 20:
yield {x:1} | tail x
                   ~

$ zq 'yield {x:1} | top x x'
zq: limit argument must be an integer at line 1, column 19:
yield {x:1} | top x x
                  ~

In recent PRs #5276 and #5296 we're making sure user-facing docs emphasize that each of these parameters should be a "compile-time constant expression that evaluates to a positive integer". However, since some users may not have read the docs, we could stand to improve the error message.