docs: fix package-wide doc errors in four math/base/special packages

[Planeshifter]

Description

What is the purpose of this pull request?

These @stdlib/math/base/special findings are not specific to the TypeScript declarations — the same errors appear consistently across each package's documentation surfaces — so they are corrected package-wide (declaration, README, REPL help, lib JSDoc, C header/source comments, and package.json as applicable):

• cscd — garbled summary "Computes the cosecant of a degree." → "Computes the cosecant of an angle measured in degrees.".
• nonfibonaccif — garbled summary "...nth non-Fibonacci single-precision floating-point number." → "...nth non-Fibonacci number as a single-precision floating-point number." (matching the double-precision sibling nonfibonacci).
• nonfibonacci — the domain Notes said "If not provided a nonnegative integer value..."; the valid domain is positive integers (n < 1 returns NaN), so reworded to "If provided either a non-integer or n < 1, the function returns NaN.".
• powf — an @example used the double-precision pi literal 3.141592653589793; use the single-precision 3.1415927410125732.

Split out from the @stdlib/math/base/special declarations PR (#12467) so the fixes apply consistently across all documentation surfaces.

Related Issues

Does this pull request have any related issues?

No.

Questions

Any questions for reviewers of this pull request?

No.

Other

Any other information relevant to this pull request?

Found via a TypeScript-declaration audit of the @stdlib/math namespace.

Checklist

Please ensure the following tasks are completed before submitting this pull request.

• Read, understood, and followed the contributing guidelines.

AI Assistance

When authoring the changes proposed in this PR, did you use any kind of AI assistance?

• Yes
• No

If you answered "yes" above, how did you use AI assistance?

• Code generation (e.g., when writing an implementation or fixing a bug)
• Test/benchmark generation
• Documentation (including examples)
• Research and understanding

Disclosure

If you answered "yes" to using AI assistance, please provide a short disclosure.

These issues were identified by an automated TypeScript-declaration audit run with Claude Code, and the fixes were prepared by Claude Code under my review.

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@stdlib-js/reviewers

[stdlib-bot]

Coverage Report

Package	Statements	Branches	Functions	Lines
math/base/special/cscd	$\color{green}186/186$ $\color{green}+100.00\%$	$\color{green}5/5$ $\color{green}+100.00\%$	$\color{green}2/2$ $\color{green}+100.00\%$	$\color{green}186/186$ $\color{green}+100.00\%$
math/base/special/nonfibonacci	$\color{green}215/215$ $\color{green}+100.00\%$	$\color{green}12/12$ $\color{green}+100.00\%$	$\color{green}2/2$ $\color{green}+100.00\%$	$\color{green}215/215$ $\color{green}+100.00\%$
math/base/special/nonfibonaccif	$\color{green}210/210$ $\color{green}+100.00\%$	$\color{green}12/12$ $\color{green}+100.00\%$	$\color{green}2/2$ $\color{green}+100.00\%$	$\color{green}210/210$ $\color{green}+100.00\%$
math/base/special/powf	$\color{red}1219/1253$ $\color{green}+97.29\%$	$\color{red}98/101$ $\color{green}+97.03\%$	$\color{red}8/10$ $\color{green}+80.00\%$	$\color{red}1219/1253$ $\color{green}+97.29\%$

The above coverage report was generated for the changes in this PR.