Attempt to fix #923 by exchanging a few =head3 directives with B<>

doc/Type/Str.pod6

@@ -404,7 +404,7 @@ Between the C<%> and the format letter, you may specify several
 additional attributes controlling the interpretation of the format. In
 order, these are:
 
-=head3 format parameter index
+B<format parameter index>
 
 An explicit format parameter index, such as C<2$>. By default,
 C<sprintf> will format the next unused argument in the list, but this
@@ -413,7 +413,7 @@ allows you to take the arguments out of order:
   sprintf '%2$d %1$d', 12, 34;      # "34 12"
   sprintf '%3$d %d %1$d', 1, 2, 3;  # "3 1 1"
 
-=head3 flags
+B<flags>
 
 One or more of:
 
@@ -455,7 +455,7 @@ precision is incremented if it's necessary for the leading "0":
   sprintf '<%#.5o>', 012345;   # "<012345>"
   sprintf '<%#.0o>', 0;        # "<>" # zero precision results in no output!
 
-=head3 vector flag
+B<vector flag>
 
 This flag tells Perl 6 to interpret the supplied string as a vector of
 integers, one for each character in the string. Perl 6 applies the
@@ -472,7 +472,7 @@ join string using something like C<*2$v>; for example:
   NYI sprintf '%*4$vX %*4$vX %*4$vX',       # 3 IPv6 addresses
           @addr[1..3], ":";
 
-=head3 (minimum) width
+B<(minimum) width>
 
 Arguments are usually formatted to be only as wide as required to
 display the given value. You can override the width by putting a
@@ -488,7 +488,7 @@ from a specified argument (e.g., with C<*2$>):
 If a field width obtained through C<*> is negative, it has the same
 effect as the C<-> flag: left-justification.
 
-=head3 precision, or maximum width
+B<precision, or maximum width>
 
 You can specify a precision (for numeric conversions) or a maximum
 width (for string conversions) by specifying a C<.> followed by a
@@ -561,7 +561,7 @@ no precision at all:
   sprintf '<%.*d>',  0, 0;          # "<>"
   sprintf '<%.*d>', -1, 0;          # "<0>"
 
-=head3 size
+B<size>
 
 For numeric conversions, you can specify the size to interpret the
 number as using C<l>, C<h>, C<V>, C<q>, C<L>, or C<ll>. For integer
@@ -587,7 +587,7 @@ used to build Perl 6:
    t           interpret integer as C type "ptrdiff_t"
    z           interpret integer as C type "size_t"
 
-=head3 order of arguments
+B<order of arguments>
 
 Normally, C<sprintf> takes the next unused argument as the value to
 format for each format specification. If the format specification uses