Fix uniform-initializer docstrings to use the range limit, not sigma

[vineethsaivs]

Proposed changes

nn.init.glorot_uniform and nn.init.he_uniform sample from a uniform distribution on [-limit, limit]: the code computes limit = gain * math.sqrt(...) and calls mx.random.uniform(-limit, limit, ...). Their docstring math, however, labels that bound as \sigma, the standard-deviation symbol:

\sigma = \gamma \sqrt{\frac{6.0}{\text{fan\_in} + \text{fan\_out}}}   # glorot_uniform
\sigma = \gamma \sqrt{\frac{3.0}{\text{fan}}}                        # he_uniform

The standard deviation of U(-limit, limit) is limit / sqrt(3), not limit, so using \sigma for the bound is misleading. This renames the symbol to \text{limit} in both docstrings so the documented quantity matches the code variable and its true meaning.

glorot_normal and he_normal are left unchanged: they sample from a normal distribution, where \sigma is the correct symbol for the standard deviation.

Documentation-only change; no code paths are affected.

Checklist

• I have read the CONTRIBUTING document
• The change is docstring text only (inside a .. math:: block), so black formatting is unchanged
• I have added tests (not applicable: documentation-only change, no behavior changed)
• I have updated the necessary documentation

[Pablosinyores]

The relabel is correct. glorot_uniform / he_uniform sample mx.random.uniform(-limit, limit, ...) where limit = gain * math.sqrt(6.0 / (fan_in + fan_out)) (resp. sqrt(3.0 / fan)), so that value is the uniform bound, not a standard deviation — \sigma was wrong here (for U[-a, a], σ is a/√3). Using \text{limit} is a nice touch since it matches the code's actual variable name.

This overlaps with your own #3831, which makes the same \sigma-is-wrong fix and additionally rewrites the prose to state the [-a, a] interval explicitly. Since both edit the same glorot/he docstring lines they'll conflict, so it's worth consolidating into one — I'd lean toward #3831 for the extra interval clarity, ideally keeping this PR's \text{limit} symbol. Flagging so one doesn't end up rotting behind the other.

[vineethsaivs]

Closing in favor of #3831, which makes the same relabel with clearer wording: it states the [-limit, limit] interval explicitly. I have folded this PR's \text{limit} symbol into #3831 so nothing here is lost. Thanks for flagging the overlap.