Added line to Apache config which explains that Apache config may be restored to default if macOS is updated

Summary:
Found a line in the Notion macOS/Xcode updates section saying that Apache httpd.conf may be reset.

Thought this was worthy of inclusion in the Apache section, so users can go through the Apache config steps again in case of error.

Test Plan: N/A

Reviewers: atul, palys-swm, ashoat

Reviewed By: atul, ashoat

Subscribers: ashoat, palys-swm, Adrian, karol-bisztyga, benschac, boristopalov

Differential Revision: https://phabricator.ashoat.com/D2361