Fixes Manishearth/namespacing-rfc#1

Fixes Manishearth/namespacing-rfc#2

Based on this comment: rust-lang#3243 (comment)

THere might be some techncial nuance missing but this should generally
get the point that users of a language with close-enough semantics have
been wanting this feature that it was added in 2003 and then improved in
2012.

I decided to forego the examples as that will be better in the
guide-level explanation.

This is pulled from rust-lang#3243 (comment)

This shifted the guide to starting from the consumer of a namespace, as
that is the majority case, and then moving into the role of a crate
owner.

This leaves out details on feature flags to not duplicate a future "why
not feature flags" section.

This tries to summarize rust-lang#3243 (comment).

Expand on the guide-level explanation

Document Python prior art

Document feature flags drawbacks

- Lack of closing paren
- A copy/paste lingering `/` separator

This was meant to pull in the guide-level explanation's reasons for this
feature but they were mostly covered.  Instead, I ended up re-wording
parts because of the semantics of `::` changes slightly the problem this
solves.  This is no longer a general project solution but specific to
when everything makes sense in the same API.  These updates are intended
to convey that.

Updaet motivation for `::` semantics

Fix a couple of typos

I can see dropping the first paragraph or moving it to another section
if we want to keep the summary more trim / focused.

Update summary to focus on `::` semantics

Co-authored-by: Eric Huss <eric@huss.org>

Co-authored-by: Eric Huss <eric@huss.org>

As this isn't an unresolved question for us, I wasn't quite sure where
to put it but felt like I should acknowledge the dilemna they are facing
somewhere.

Update from recent discussions

Co-authored-by: Oli Scherer <github35764891676564198441@oli-obk.de>

…tional-namespaces.md