x/exp/rand: update documentation to direct users to math/rand/v2

[meling]

According to #61716, the x/exp/rand package should be tagged and deleted from the main branch.

I see that it was recently tagged with v0. However, I suggest updating the documentation to direct users to math/rand/v2. Perhaps also deprecating the package (not sure about the policy on full package deprecation).

Rationale:

• People may accidentally use x/exp/rand instead of math/rand/v2, even if they are on Go 1.22 or higher.
• I caught one such case in code review just now. I went looking at x/exp/rand, wondering if this was a precursor of math/rand/v2, but nothing was mentioned about it in the documentation, adding to my confusion.
• I suspect gopls' heuristic for selecting which package to import is at play... (see also x/tools/gopls: picks random import path not in go.mod when wanted import path for package is in go.mod #61208).

[dr2chase]

@rsc @griesemer

[gabyhelp]

Related Issues

• proposal: Go 2: move math/rand to x/math/rand #24800 (closed)
• x/tools/cmd/goimports: prefer rand/v2 where applicable #66407 (closed)
• x/website: `math/rand/v2` was added in Go 1.22, not Go 1.23 #67322 (closed)
• math/rand: deprecate Read #20661 (closed)
• x/pkgsite: search for math/rand should answer with math/rand/v2 too #66452 (closed)

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[gopherbot]

Change https://go.dev/cl/644475 mentions this issue: rand: deprecate in favor of math/rand/v2