doc/langref: document the blank identifier

[perillo]

The Go Language Specification documents the _ identifier, called the blank identifier: https://go.dev/ref/spec#Blank_identifier.

However the Zig Language Reference only says:

"The `_ = C;` syntax is a no-op reference to the identifier `C`"

inside an example.

[Vexu]

It's also documented in the paragraph above the example:

The code sample below uses the std.testing.refAllDecls(@This()) function call to reference all of the containers that are in the file including the imported Zig source file. The code sample also shows an alternative way to reference containers using the _ = C; syntax. This syntax tells the compiler to ignore the result of the expression on the right side of the assignment operator.

Related #4164.