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

generate/main.go

@@ -1,3 +1,6 @@
+// Package generate provides programmatic access to genqlient's functionality,
+// and documentation of its configuration options.  For general usage
+// documentation, see github.com/Khan/genqlient.
 package generate
 
 import (
@@ -50,6 +53,9 @@ See https://github.com/Khan/genqlient for full documentation.
 `)
 }
 
+// Main is the command-line entrypoint to genqlient; it's equivalent to calling
+// `go run github.com/Khan/genqlient`.  For lower-level control over
+// genqlient's operation, see Generate.
 func Main() {
 	exitIfError := func(err error) {
 		if err != nil {