[time.format] Fix description of - insertion and examples.

 * Place :% in correct order in example.
 * Ensure that format specifier is at start of format-specs as required
   by the grammar.
 * Clarify that 'leftmost' means 'initial', and doesn't mean the final
   format specifier in an RTL locale.

Fixes #3810.

Author: zygoloid

source/time.tex

@@ -10496,12 +10496,12 @@
 an \tcode{hh_mm_ss} object \tcode{h} for which \tcode{h.is_negative()} is \tcode{true},
 is equivalent to the output of the corresponding positive value,
 with a \tcode{\exposid{STATICALLY-WIDEN}<charT>("-")} character sequence
-placed before the replacement of the leftmost conversion specifier.
+placed before the replacement of the initial conversion specifier.
 \begin{example}
 \begin{codeblockdigitsep}
-cout << format("{%:T}", -10'000s);          // prints: \tcode{-02:46:40}
+cout << format("{:%T}", -10'000s);          // prints: \tcode{-02:46:40}
 cout << format("{:%H:%M:%S}", -10'000s);    // prints: \tcode{-02:46:40}
-cout << format("{:minutes %M, hours %H, seconds %S}", -10'000s);
+cout << format("minutes {:%M, hours %H, seconds %S}", -10'000s);
                                             // prints: \tcode{minutes -46, hours 02, seconds 40}
 \end{codeblockdigitsep}
 \end{example}