revsets.md: document the all: prefix

While this is arguably not part of the revset language, this
is a likely place for a user to look.

See https://discord.com/channels/968932220549103686/968932220549103689/1228065431281995837

docs/config.md

@@ -328,24 +328,16 @@ Can be customized by the `format_short_signature()` template alias.
 
 ### Allow "large" revsets by default
 
-Certain commands (such as `jj rebase`) can take multiple revset arguments, and
-each of these may resolve to one-or-many revisions. By default, `jj` will not
-allow revsets that resolve to more than one revision — a so-called "large
-revset" — and will ask you to confirm that you want to proceed by
-prefixing it with the `all:` modifier.
+By default, `jj` requires the `all:` prefix for many commands to accept a revset
+that expands to multiple arguments (AKA "large revsets") as a safety check. See
+[the docs for `all:`](revsets.md#the-all-prefix) for details. 
 
-For instance, to add a new parent `abc` to the commit `xyz`, you may use `jj
-rebase`:
-
-```
-jj rebase -r xyz -d "all:xyz-" -d "abc"
-```
-
-`jj` requires the `all:` prefix for the above command. However, you may disable
-this behavior by setting `ui.always-allow-large-revsets` to `true`:
+However, you may disable this check by setting `ui.always-allow-large-revsets`
+to `true`. Then, `jj` will no longer require the `all:` prefix to revsets for
+any commands that otherwise would.
 
 ```toml
-ui.always-allow-large-revsets = true
+ui.always-allow-large-revsets = true  # Assume `all:` prefix whenever it would make a difference
 ```
 
 ## Pager

docs/revsets.md

@@ -70,6 +70,38 @@ only symbols.
 You can use parentheses to control evaluation order, such as `(x & y) | z` or
 `x & (y | z)`.
 
+### The `all:` prefix
+
+An `all:` prefix before a revset expression does not change its meansing.
+Strictly speaking, it is not part of the revset language. The notation is
+similar to the prefixes allowed before [string patterms](#string-patterns).
+
+Certain commands (such as `jj rebase`) can take multiple revset arguments, and
+each of these may resolve to one-or-many revisions. By default, `jj` will not
+allow revsets that resolve to more than one revision — a so-called "large
+revset" — and will ask you to confirm that you want to proceed by
+prefixing it with the `all:` modifier.
+
+Alternatively, you can set the `ui.always-allow-large-revsets` option to `true`.
+
+For example, `jj rebase -r w -d xyz+` will rebase `w` on top of the child of
+`xyz` as long as `xyz` has exactly one child.
+
+If `xyz` has more than one child, the `all:` prefix is *not* specified, and
+`ui.always-allow-large-revsets` is `false` (the default), `jj rebase -r w -d
+xyz+` will return an error.
+
+If `ui.always-allow-large-revsets` was `true`, the above command would act as if
+`all:` was set (see the next paragraph).
+
+With the `all:` prefix, `jj rebase -r w -d all:xyz+` will make `w` into a merge
+commit if `xyz` has more than one child. The `all:` prefix confirms that the
+user expected `xyz` to have more than one child.
+
+A more useful example: if `w` is a merge commit, `jj rebase -s w -d all:w- -d
+xyz` will add `xyz` to the list of `w`'s parents.
+
+
 ## Functions
 
 You can also specify revisions by using functions. Some functions take other