Correct TOMS 748 root bracketing docs

[mpwaser]

Fixes #1166.

Summary

This is a documentation-only update for the TOMS 748 root-finding docs.

The parameter description now says that the initial bounds must bracket at
least one root, rather than a single root. The f(a) * f(b) < 0 precondition
remains documented immediately below that text.

Rationale

• TOMS Algorithm 748 is described as enclosing a zero of a continuous function
  (Alefeld, Potra, and Shi, ACM TOMS 21(3), 1995, DOI:
  10.1145/210089.210111).
• Boost.Math's implementation enforces a sign-changing bracket, not uniqueness:
  toms748_solve rejects same-sign endpoint values, and detail::bracket
  updates the interval by comparing endpoint signs.
• With continuous f, the endpoint sign change guarantees at least one
  sign-changing root in the interval. If all roots in the interval are simple
  crossings, this implies an odd number of roots, but additional
  even-multiplicity roots may also lie in the same bounds, so uniqueness is not
  required.

Changes

• update the Quickbook source in doc/roots/roots_without_derivatives.qbk
• update the corresponding pregenerated HTML page under doc/html

Validation

• inspected the issue discussion and existing TOMS 748 precondition wording
• inspected include/boost/math/tools/toms748_solve.hpp
• compiled and ran a local smoke test showing:
  • a sign-changing interval containing two distinct roots, one crossing root
    and one even-multiplicity root, is accepted
  • an interval containing two simple roots but no endpoint sign change is
    rejected
• checked Boost.Math and Boost superproject contribution/template files; no PR
  template is present in the repo or organization-level .github repo
• checked official Boost requirements/submission guidance and Math README docs
  build instructions
• bootstrapped b2 and initialized the required Boost documentation tool
  submodules/dependencies
• ran ../../../b2 -q --user-config=<repo>/user-config.jam from libs/math/doc
• ran git diff --check