Add documentation for drop_value sender adaptor

[msimberg]

No description provided.

[codacy-production]

Coverage summary from Codacy

See diff coverage on Codacy

Coverage variation	Diff coverage
✅ +0.08% (target: -1.00%)	✅ 100.00% (target: 90.00%)

Coverage variation details

	Coverable lines	Covered lines	Coverage
Common ancestor commit (922a33c)	18260	13777	75.45%
Head commit (0248507)	18259 (-1)	13791 (+14)	75.53% (+0.08%)

Coverage variation is the difference between the coverage for the head and common ancestor commits of the pull request branch: <coverage of head commit> - <coverage of common ancestor commit>

Diff coverage details

	Coverable lines	Covered lines	Diff coverage
Pull request (#1287)	1	1	100.00%

Diff coverage is the percentage of lines that are covered by tests out of the coverable lines that the pull request added or modified: <covered lines added or modified>/<coverable lines added or modified> * 100%

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[biddisco]

LGTM

[aurianer]

LGTM thanks!