Fix docstring formatting/signature bugs; drop broken export

[AronT-TLV]

Summary

Phase 7.1 — same docstring formatting audit already done in Linear_Algebra (#20), applied here. Tutorial pages (docs/src/*.md) had no hand-typed duplicate signatures to fix — they already pull docstrings via @autodocs.

• plot_parabola: docstring claimed p::Polynomial; the real parameter is untyped since the function is called with both a Polynomials.Polynomial and a Symbolics expression.
• plot_hyperbola: removed a dead orphaned docstring describing a stale, no-longer-existing signature (plot_hyperbola(n::Integer)), removed a stray leftover artifact line, and fixed a formula typo ("+ b" instead of "+ k", inconsistent with the surrounding text and the actual code).
• triangle_area_perim: a blank line + indented description text was accidentally forming a second indented code block, so the description rendered as monospace code instead of prose.
• accrued_apr, accrued: dropped the function keyword prefix from docstring signature lines, matching the convention already applied in Linear_Algebra and documented in the documenter-jl-conventions skill.
• Math_Foundations.jl: removed plot_hyperbola_axes from the export list — no such function exists (only plot_hyperbola_axes_varx/_direct do). Julia tolerates exporting an undefined name at load time, so this was silently broken rather than erroring.

Test plan

• julia --project=docs docs/make.jl builds cleanly with no errors
• typos passes on both edited source files
• Verified in the built HTML that plot_parabola, plot_hyperbola, triangle_area_perim, accrued_apr, accrued all render correctly with matching signatures/prose, and a full-page scan found no mangled identifiers
• Full test suite passes (Pkg.test()): 75/75

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