docs: note that deno task ** recurses into node_modules

[bartlomieju]

Adds a caution note to the deno task glob expansion docs clarifying that,
because globstar is enabled by default, ** recurses into all descendant
directories including node_modules. This surprises users coming from shells
where ** is not recursive by default: a task like deno check **/*.ts can
expand to far more files than the same command typed in the terminal, leading to
errors like Argument list too long or type-checking unintended dependency
files.

The note points to running deno check / deno fmt without glob arguments plus
the exclude config (which excludes node_modules by default), or disabling
recursion per-task with shopt -u globstar.

Prompted by denoland/deno#31910.