Clean up godoc (#82)

## Summary:
Before open-sourcing, we want to make sure that (a) GoDoc looks
reasonable, and (b) everything in the API is something we want to commit
to.  In this commit, I do some miscellaneous cleanup on both fronts;
this does involve a few breaking changes to the programmatic API (better
now than once it has users).  In future commits, I'll likely move the
documentation for `genqlient.yaml` and `@genqlient` to clearer places,
and make `GenqlientDirective` private, such that GoDoc is really only
for programmatic users.

Fixes #25.

Issue: https://github.com/Khan/genqlient/issues/25

## Test plan:
make check


Author: benjaminjkraft

Reviewers: dnerdy, benjaminjkraft, jvoll, aberkan, MiguelCastillo, mahtabsabet

Required Reviewers: 

Approved By: dnerdy, jvoll

Checks: ✅ Test (1.17), ✅ Test (1.16), ✅ Test (1.15), ✅ Test (1.14), ✅ Lint, ✅ Test (1.17), ✅ Test (1.16), ✅ Test (1.15), ✅ Test (1.14), ✅ Lint

Pull Request URL: https://github.com/Khan/genqlient/pull/82

main.go

@@ -1,4 +1,9 @@
-// package main is at the root to allow "go run github.com/Khan/genqlient"
+// genqlient is a GraphQL client generator for Go.
+//
+// To run genqlient:
+//	go run github.com/Khan/genqlient
+// For programmatic access, see the "generate" package, below.  For
+// user documentation, see github.com/Khan/genqlient.
 package main
 
 import (