Documentation fixes for paste.Rd and substr.Rd#223

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aakritixyz wants to merge 3 commits intor-devel:mainfrom

aakritixyz:doc-fixes-paste-substr

Commits on Jan 1, 2026

Doc: Clarify paste0 efficiency and substr zero-length behavior
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Commits on Jan 6, 2026

Clarify examples: recycle0 behavior and zero-length vs empty strings
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Remove redundant recycle0/collapse examples from paste.Rd
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