D modules

[pascal111]

When I'm searching for "toUpper" and "toLower" functions that 
string type uses, I confused when I reached the module 
"std.string". In the first section of its page 
"https://dlang.org/phobos/std_string.html" I didn't found 
functions I'm searching for, but when I pressed ctrl+f and typed 
function names, I found 'em in another section of the page, but I 
didn't understand if I have to add something to module name 
"std.string" or not, and I don't know how modules divided nor its 
sections way in D. I hope someone clarifies this obscure point to 
me and how can I search correctly for sought functions.