[PATCH] doc: Fix description of relative extension "directory" setting

Hi,

Since PostgreSQL 18 (commit 81eaaa2c41d), the documentation of the
extension control file "directory" parameter says a relative path is
"relative to the directory where the control file was found".
However, that doesn't match the code: a relative path is resolved
against SHAREDIR,
or against the extension_control_path element in which the control
file was found, i.e. the parent of that "extension" directory.

Repro steps on master:

/tmp/repro/extension/dir_doc_probe.control:
default_version = '1.0'
directory = 'scripts'
relocatable = true

/tmp/repro/scripts/dir_doc_probe--1.0.sql:
CREATE TABLE dir_doc_probe_result (origin text);
INSERT INTO dir_doc_probe_result VALUES ('installation base');

/tmp/repro/extension/scripts/dir_doc_probe--1.0.sql:
CREATE TABLE dir_doc_probe_result (origin text);
INSERT INTO dir_doc_probe_result VALUES ('control-file directory');

BEGIN;
SET LOCAL extension_control_path = '/tmp/repro';
CREATE EXTENSION dir_doc_probe;
SELECT origin FROM dir_doc_probe_result;
origin
-------------------
installation base
(1 row)
ROLLBACK;

The attached patch restores the pre-18 wording, extended to cover
extension_control_path, and clarifies the extension_control_path
description accordingly.