docs: output growth with nesting depth and ordered_json objects with many keys (#6118)

untrusted_input.md did not say that indented dump() and flatten() produce
results that grow with the nesting depth times the size of the value (80 KB
of input give 600 MB of dump(1)), nor that the binary readers take
quadratic time for objects with many keys when they read into
ordered_json. Add a warning to "Nesting depth", a section "Objects with
many keys in ordered_json", and both points to the checklist.

The warning in ordered_map.md still said that parsing JSON text into
ordered_json is quadratic; that is no longer true. Name the cases that
still are (filling an object key by key, the binary readers) and replace
the table with current measurements.

dump.md and flatten.md called their complexity linear in the value; it is
linear in the result, which can be larger by a factor of the depth.

Fixes #6108. Refs #6104.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

docs/mkdocs/docs/api/basic_json/dump.md

@@ -58,7 +58,9 @@ is not UTF-8 encoded and `error_handler` is set to `strict`
 
 ## Complexity
 
-Linear.
+Linear in the size of the result. With an `indent`, every line starts with up to `indent` characters per nesting level,
+so the result can be larger than the value by a factor of its nesting depth (see
+[Parsing untrusted input](../../features/parsing/untrusted_input.md#nesting-depth)).
 
 ## Notes