Update 'More efficient string construction' documentation

Consider further updating the QString overview documentation to guide people towards operator""_s in the StringLiterals namespace.

• Initializing a string starts with a normal const char * constructor, and goes on explaining some rather esoteric ways of initializing a string. operator""_s or operator""_L1 is not mentioned. I think it should be at least mentioned alongside const char * constructor, optionally with a reference to More Efficient String Construction.

• More Efficient String Construction starts with mentioning QStringLiteral. It should arguably mention operator""_s instead.

• More Efficient String Construction also mentions QLatin1StringView. Consider giving an example using the operator""_L1.

• QStringLiteral documentation should also mention operator""_s as alternative.

Related:

• More Efficient String Construction could do a better job giving the tradeoffs (runtime costs vs memory size).

• The code example in More Efficient String Construction doesn't make any sense. Variable foo always starts with vector<.

• (minor) Remove reference to 'In 4.6, ' .