PUPPETEER_DOWNLOAD_HOST poorly named & documented

[abentley]

Steps to reproduce

Tell us about your environment:

• Puppeteer version: 1.10.0
• Platform / OS version: Ubuntu 18.04
• URLs (if applicable):
• Node.js version:

What steps will reproduce the problem?
PUPPETEER_DOWNLOAD_HOST is not a hostname, rather it is a URL prefix. Hostnames do not include protocol, nor do they indicate path prefix. This name makes the variable less discoverable for users who need to override protocol and / or path.

The documentation:
https://github.com/GoogleChrome/puppeteer/blob/master/docs/api.md#environment-variables
"overwrite host part of URL that is used to download Chromium"

Again, this makes it seem like the PUPPETEER_DOWNLOAD_HOST cannot override the protocol or path prefix, when in fact, it can. It may also lead to an incorrect value being supplied, e.g. "my-mirror.com" rather than "http://my-mirror.com".

At one point in the discussion of issue #951, PUPPETEER_DOWNLOAD_CHROMIUM_URL_PREFIX was considered as the name. This is verbose, but far more descriptive of how the variable actually functions.

Please include code that reproduces the issue.

N/A

What is the expected result?
N/A

What happens instead?
N/A

[aslushnikov]

@abentley These are all fair concerns. I don't think changing name worth it at this point; we can do a better job documenting the behavior though.