docs: some small updates after beta 39+

[jemgillam]

Description

Slowly tweaking some documentation to align with changes made in grafast beta 39 onwards

ref https://github.com/graphile/crystal/blob/main/postgraphile/postgraphile/CHANGELOG.md#500-beta39

• "modifier step" now becomes "Modifier class"
• dropping the "Step" suffix as needed
• "wherePlan" and "havingPlan" have been removed from pgUnionAll and pgSelect
• I've assumed fieldArgs.get becomes fieldArgs.getRaw in our simple examples
• $step.placeholder($step, codec) becomes step.sqlValueWithCodec(value, codec)

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[jemgillam]

This is good to be reviewed and merged

[benjie]

Thanks!

[benjie]

@jemgillam I'm not sure this belongs here.

[jemgillam]

It was the following, I assume you thought it was an easy self-explanatory example

const $users = usersResource.find();
const tbl = $users.alias;

const $username = fieldArgs.getRaw("username");
const frag = $users.placeholder($username, TYPES.citext);

$users.where(sql`${tbl}.username = ${frag}`);

Remove the example if it's too complicated?