docs: fix crashes on searching documentation of various expressions

[vtjnash]

Fix a crash when calling @doc on non-document-able expressions so that docs (or lack thereof) can be successfully printed. For example, looking up docs on sad faces, doesn't crash and cause sad faces after this change:

help?> :()
  Expr(head::Symbol, args...)

Details: expands astname to handle valid expression types explicitly, removes the unused, undocumented @var export from Base.Docs, and updates the error message to be clearer.

🤖 Generated with some assistance from Claude Code.

[vtjnash]

@nanosoldier runtests(ALL, configuration = (goal=:load,), vs_configuration = (goal=:load,))

[nanosoldier]

The package evaluation job you requested has completed - possible new issues were detected.
The full report is available.

Report summary

❗ Packages that crashed

1 packages crashed on the previous version too.

✖ Packages that failed

10 packages failed only on the current version.

• There were unidentified errors: 6 packages
• Test duration exceeded the time limit: 4 packages

671 packages failed on the previous version too.

~ Packages that at least loaded

10 packages successfully loaded only on the current version.

• Other: 10 packages

9398 packages successfully loaded on the previous version too.

➖ Packages that were skipped altogether

1 packages were skipped only on the current version.

• Package could not be installed: 1 packages

938 packages were skipped on the previous version too.

[vtjnash]

Good. Only 2 pkgeval issues, both of them due to bugs (documenting the wrong object)

https://github.com/QuEraComputing/Bloqade.jl/blob/bb01bc4b5b1c437c0a3e7913be5185c2c3fb3e63/lib/BloqadeWaveforms/src/waveform.jl#L658 (trying to document the anonymous function function (..) end of one argument instead documents the argument ..)

Mis-parsing of const expression (documents only LEFT and returns :LEFT instead for 1:4):
https://s3.amazonaws.com/julialang-reports/nanosoldier/pkgeval/by_hash/4ca97dc_vs_05c07e1/Game2048Core.primary.log
https://github.com/xiaodaigh/Game2048Core.jl/blob/0eaf7ed4b1a7f898a9a10ba9aaa2a480712db8b8/src/bitboard/bitboard.jl#L15

"""
Bsed on the benchmarks I have done. It's quicker to precompute the results for all rows and make a
set of lookup table that looks up the result of moving left or right.

One of the Rust repositories have taken this approach.
"""
const LEFT, LEFT_REWARD, RIGHT, RIGHT_REWARD = 1:4

[vtjnash]

Submitted a PR to fix Bloqade, since I cannot think of a valid reason that we should continue to support this syntax to document the first argument instead of the function: QuEraComputing/Bloqade.jl#708

I'm inclined to merge this soon, then add proper support of documenting multiple assignments as a blocking feature for v1.13 in a followup PR