docs: Clarify coalesce behaviour in join_asof

[MarcoGorelli]

closes #18089

Join asof is like a left-join, so I think the argument (and description) can be simplified too, right? it's either true or false, nothing join-dependent

[codecov]

Codecov Report

Attention: Patch coverage is 75.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 80.20%. Comparing base (c88da1f) to head (5ee4bb2).
Report is 2 commits behind head on main.

Files	Patch %	Lines
crates/polars-python/src/lazyframe/general.rs	75.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #18273      +/-   ##
==========================================
- Coverage   80.21%   80.20%   -0.02%     
==========================================
  Files        1500     1500              
  Lines      198897   198896       -1     
  Branches     2837     2837              
==========================================
- Hits       159547   159516      -31     
- Misses      38822    38852      +30     
  Partials      528      528              

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