docs: caveats of multiple package managers

[jpbnetley]

I had an issue where pnpm was installed with corepack, but I would have a version mismatch when comparing the package manager in the package.json and the pnpm version in the terminal.

I created an issue with corepack: nodejs/corepack#659

After some debugging, we determined that I had 2 versions of pnpm installed.
One via corepack, and another via globally from npm.

See here as reference: nodejs/corepack#659 (comment)

I was then suggested to maybe add documentation to warn against this odd behaviour here: nodejs/corepack#659 (comment)

I am considering adding some information regarding this here: https://github.com/pnpm/pnpm.io/blob/main/docs/installation.md
on line 97.

:::warning

Having multiple package manager versions installed can cause an incorrect version of a package manager being used, regardless of the version that is specified in the package.json.

To verify there are not multiple version installed, see this [corepack issue 659](https://github.com/nodejs/corepack/issues/659#issuecomment-2658982866).

:::

[zkochan]

You had pnpm v9 installed. With pnpm v10 you wouldn't have noticed any issue as it would switch to the right pnpm version automatically. So, I don't think a notice is needed.

[jpbnetley]

For me that is where the confusion came in.
Because the package.json specified v10.
And I happen to have a pnpm version installed with corepack and globally.
It would always use v9, even though the package json specified v10.
(as we discovered here: nodejs/corepack#659 (comment))
For me, that was confusing.

I think in my case what caused this confusion is corepack could not install the new version after throwing a key id mismatch error. (see nodejs/corepack#612 for more info)
And one of the suggested fixes was to install pnpm globally: nodejs/corepack#612 (comment)

Eventually I noticed the version miss match and didn't understand why the version specified in the package.json was not used.

But if you think the info is redundant, I can close the issue and docs pr.
Let me know.

Thank you for taking the time to go through the issue :)