docs: add ROLLUP_WATCH envar to docs

[shellscape]

This PR contains:

• bugfix
• feature
• refactor
• documentation
• other

Are tests included?

• yes (bugfixes and features will not be merged without tests)
• no

Breaking Changes?

• yes (breaking changes will not be merged unless absolutely necessary)
• no

List any relevant issue numbers:

Description

Small documentation update to include ROLLUP_WATCH envar. fixes #2711.

[codecov]

Codecov Report

Merging #3048 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #3048   +/-   ##
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  Coverage   88.68%   88.68%           
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  Files         165      165           
  Lines        5708     5708           
  Branches     1743     1743           
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  Hits         5062     5062           
  Misses        388      388           
  Partials      258      258

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[lukastaegert]

Alternatively, the ROLLUP_WATCH environment variable may be set to true to enable watch mode

Looking at the code, this is not correct. Instead while in watch mode, Rollup CLI adds the new environment variable, which can be checked by plugins to exhibit different behaviour.

rollup/cli/run/index.ts

Line 86 in 6b2b650

if (command.watch) process.env.ROLLUP_WATCH = 'true';

It can be argued that this is a weird way of doing it and a property on this.meta of the plugin context would be a better place to make this information available but this is how it is implemented. How about

While in watch mode, the ROLLUP_WATCH environment variable will be set to "true" by Rollup's command line interface and can be checked by plugins and other processes.

[shellscape]

@lukastaegert that suggestion is good by me. are you OK with me updating master directly with that change?

[lukastaegert]

Sure, please go ahead!

[shellscape]

Done!