DOC: Fix and improve docstrings across entropy, fractal, and utils modules

[raphaelvallat]

This PR corrects several inaccuracies in docstrings and improves documentation clarity throughout the codebase.

Bug fixes in formulas and descriptions

• perm_entropy: Fix formula — log2(π) → log2 p(π) (argument to log must be the probability, not the permutation label)
• svd_entropy: Fix normalization factor — log2(order!) → log2(order) (copy-paste from perm_entropy); fix "permutation entropy" → "SVD entropy" in same line
• spectral_entropy: Fix normalize description — psd.size → number of frequency bins (these differ for N-D arrays); clarify nperseg=None behaviour
• detrended_fluctuation: Fix broken RST — stray double-backtick after :math: directives in Returns; fix "exponant" typo; replace internal entropy.utils._log_n(...) reference with a plain-English description;
  improve grammar in Returns

Typos and wording

• lziv_complexity: Remove misleading "(binary)" from title; fix "begining" → "beginning"; "character" → "characters"
• sample_entropy: "inferior to" → "less than"
• hjorth_params: "EntroPy" → "AntroPy"
• num_zerocross, hjorth_params: Fix .. versionadded: → .. versionadded:: (single colon broke the RST directive)

New content

• higuchi_fd: Add mathematical derivation of L_m(k) and L(k) to Notes, matching the actual implementation (sum runs to N_m − 1, consistent with the upstream MNE-features source); add Esteller et al. (2001) and
  Paivinen et al. (2005) references cited by MNE-features

utils cleanup

• Fix _xlog2x → _xlogx in all (function was never named _xlog2x)
• Standardise _log_n docstring to NumPy style; fix return type (np.ndarray of int64, not "list of integers"); correct "< max_n" → "<= max_n"

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (ab8c158) to head (440b207).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

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  Coverage   100.00%   100.00%           
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  Files            4         4           
  Lines          302       302           
  Branches        45        45           
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