Rollup of 11 pull requests

[tmandry]

Successful merges:

• Improve documentation for std::{f32,f64}::mul_add #77027 (Improve documentation for std::{f32,f64}::mul_add)
• Make the kernel_copy tests more robust/concurrent. #79375 (Make the kernel_copy tests more robust/concurrent.)
• Add long explanation for E0212 #79639 (Add long explanation for E0212)
• Add tracking issue template for library features. #79698 (Add tracking issue template for library features.)
• Dogfood str_split_once() #79809 (Dogfood str_split_once())
• Clarify the 'default is only allowed on...' error #79851 (Clarify the 'default is only allowed on...' error)
• Update const-fn doc in unstable-book #79858 (Update const-fn doc in unstable-book)
• Clarify that String::split_at takes a byte index. #79860 (Clarify that String::split_at takes a byte index.)
• Fix small typo in wrapping_shl documentation #79871 (Fix small typo in wrapping_shl documentation)
• Make search results tab and help button focusable with keyboard #79896 (Make search results tab and help button focusable with keyboard)
• Use Symbol for inline asm register class names #79917 (Use Symbol for inline asm register class names)

Failed merges:

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