docs: properly explain conversions in nix eval --json/--raw options.

jade: I rewrote this PR to be consistent between nix-instantiate and nix
eval. It turns out that nix eval *doesn't* copy to store with `--json`,
whereas nix-instantiate does. Wat.

Closes: https://github.com/lix-project/lix/pull/17
Co-authored-by: tyberiusprime <tyberiusprime@noreply.git.lix.systems>

Change-Id: Id22deec1cee0fed3bd5689567869b70bab26bae5

doc/manual/src/command-ref/nix-instantiate.md

@@ -107,20 +107,27 @@ See that section for complete details (`nix-build --help`), but in summary, a pa
     > This option can cause non-termination, because lazy data
     > structures can be infinitely large.
 
-  - `--raw`
+  - `--raw`\
     When used with `--eval`, the result must be coercible to a string, i.e.,
-    something that can be converted using `${...}`. The output is
-    printed exactly as-is, with no quotes, escaping, or trailing newline.
+    something that can be converted using `${...}`.
+
+    Integers will always generate an error when output via `--raw`, regardless of
+    [`coerce-integers`](../contributing/experimental-features.md#xp-feature-coerce-integers) being enabled, to avoid ambiguity.
+
+    The output is printed exactly as-is, with no quotes, escaping, or trailing
+    newline.
 
   - `--json`\
     When used with `--eval`, print the resulting value as an JSON
-    representation of the abstract syntax tree rather than as a Nix expression.
+    representation of the resulting value rather than as a Nix expression.
+
+    The conversion behaviour, if `--strict` is passed, is the same as
+    [`builtins.toJSON`](../language/builtins.md#builtins-toJSON).
 
   - `--xml`\
     When used with `--eval`, print the resulting value as an XML
-    representation of the abstract syntax tree rather than as a Nix expression.
-    The schema is the same as that used by the [`toXML`
-    built-in](../language/builtins.md).
+    representation of the resulting value rather than as a Nix expression.
+    The schema is the same as that used by [`builtins.toXML`](../language/builtins.md#builtins-toXML).
 
   - `--read-write-mode`\
     When used with `--eval`, perform evaluation in read/write mode so

lix/nix/eval.md

@@ -61,12 +61,20 @@ result on standard output.
 
 * By default, the evaluation result is printed as a Nix expression.
 
-* With `--json`, the evaluation result is printed in JSON format. Note
-  that this fails if the result contains values that are not
-  representable as JSON, such as functions.
+* With `--json`, the evaluation result is printed in JSON format.
 
-* With `--raw`, the evaluation result must be a string, which is
-  printed verbatim, without any quoting.
+  The conversion behaviour is the same as
+  [`builtins.toJSON`](../../language/builtins.md#builtins-toJSON) except that
+  paths are printed as-is without being copied to the Nix store.
+
+* With `--raw`, the result must be coercible to a string, i.e.,
+  something that can be converted using `${...}`.
+
+  Integers will always generate an error when output via `--raw`, regardless of
+  [`coerce-integers`](../../contributing/experimental-features.md#xp-feature-coerce-integers) being enabled, to avoid ambiguity.
+
+  The output is printed exactly as-is, with no quotes, escaping, or trailing
+  newline.
 
 * With `--write-to` *path*, the evaluation result must be a string or
   a nested attribute set whose leaf values are strings. These strings