clarify site homepage practice, could still use the is-homepage symbol

[narration-sd]

Description

I had extra fun getting a homepage Single to just work, on a very simple Craft3 site.

• It seems to deviate from the default value provided by CP for the Section, as only index is going to be findable , as the Twig template, while the default inserted was as elsewhere, _entry. Which also works elsewhere, when it's not on url / as in next item. Possibly this behavior is just a matter of _somename templates being blocked unless they have a folder prefix?
• it suffices to declare the Section url as /, but this leads to questions about function of handle
• on such a / url'd Section, there is no longer any value for entry available from the template. Thus I had to look it up with ```{% set entry = craft.entries().section('homePage').one %}`` which seems a bit rough for the polity.

So, homepage marking checkbox back in Section dialog, or have you intended other ways?

Steps to reproduce

1. Set up a Section with any Name/Handle, leaving the default _entry, and set url to /, then fill in your fields def, and make a simple template/_entry.html to use a few fields, {{ entry.body }} for example.
2. Try to access site url alone, and get a 404ish screen
3. change the section def to use index instead of _entry, and rename templates/_entry to index
4. home page tries to come up, but faults out on entry being null, not saying anything as usual about where that might be in Twig (see Better responses if false Asset Transform handle -- and from Twig errors in general #1454)
5. define own entry in index.html, as in Description
6. Now / home page functions, but on Craft 3 there's no indication in Section or Sections listing that it actually is the homepage.

Additional info

PHP version 7.1.2-3+deb.sury.orgxenial+1
Database driver & version MySQL 5.7.17-0ubuntu0.16.04.1
Image driver & version GD 7.1.2-3+deb.sury.orgxenial+1
Craft edition & version Craft Personal 3.0.0-beta.6
Yii version 2.0.11.2
Twig version 2.1.0
Guzzle version 6.2.1
Imagine version 0.7-dev
Plugins: none, bare test site

[narration-sd]

One thing about this item cleared up, perhaps literally so.

I was playing with Sites this morning, to wake up after a long week, and noticed the fine print.

In the uri field of a Section, before you fill it in, there is actually a statement to leave it blank if this is the homepage - multiply so if you have Sites. This is quite faint -- another case probably of what looks good on a Mac Retina screen coming out a bit different on a non-IPS laptop. You can think about it.

As a way to go, though, it works. It might still be nice if you had a symbol then, perhaps in the listing page for Sections, and in the Section page once it's been saved, noting it's a home page -- and this could have a tooltip announcing it is so because uri is blank.