WASM: document platform-dependent intp overflow boundary (#929)

[kp992]

Closes #929.

Documents the np.iinfo(np.intp).max overflow boundary in comb_jit, num_compositions_jit, simplex_grid, and k_array_rank_jit, noting it is 2³¹−1 on wasm32 (JupyterLite) vs 2⁶³−1 on 64-bit platforms. No code changes.

Grep sweep of all intp/int_ sites in the library confirms every use is an index or array size — none need widening to int64.

[coveralls]

coverage: 90.847%. remained the same — kp992:wasm-929-intp-docs into QuantEcon:main

[kp992]

Verified this on https://notebook.link/@anutosh491/numba-ecosystem.

On my local machine:

% python 
Python 3.11.9 | packaged by conda-forge | (main, Apr 19 2024, 18:34:54) [Clang 16.0.6 ] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import numpy as np
>>> np.iinfo(np.intp).max, 2**63-1
(9223372036854775807, 9223372036854775807)