OpenGL Reference pages conflict with specifications for `atan` undefined behavior

The Reference pages for `atan` indicate that "the result is undefined if x=0".

https://registry.khronos.org/OpenGL-Refpages/gl4/html/atan.xhtml

However, the GLSL specification does not match, and for atan it states "results are undefined if x and y are both 0."

https://registry.khronos.org/OpenGL/specs/gl/GLSLangSpec.4.60.pdf

(The Reference page phrasing is also somewhat vague because it has a paragraph specifically about "the second overload"—the one containing an x parameter—but the text about undefined results is not contained in this paragraph.)

FWIW, the SPIR-V definition of Atan2 matches the GLSL specification's text (undefined when x and y are both 0), not the Reference pages text (x=0).

Compare the `atan` Reference page with the GLSL 4.60 specification and the repository's source for that page. Update the undefined-behavior wording to distinguish the overloads and match the specification, then verify the generated HTML reflects the correction.

Written by the indexing model from the issue text.