Docs: Be more clear when specifying valid formats for strings

- Use the word letter rather than character to refer to letters ;) when trying to specify that only letters and numbers can be used, and not ANY character...
- Small corrections

Fixes #29821

Signed-off-by: Timothy Hobbs <timothy@hobbs.cz>

docs/reference/commandline/tag.md

@@ -29,13 +29,13 @@ by a registry hostname. The hostname must comply with standard DNS rules, but
 may not contain underscores. If a hostname is present, it may optionally be
 followed by a port number in the format `:8080`. If not present, the command
 uses Docker's public registry located at `registry-1.docker.io` by default. Name
-components may contain lowercase characters, digits and separators. A separator
+components may contain lowercase letters, digits and separators. A separator
 is defined as a period, one or two underscores, or one or more dashes. A name
 component may not start or end with a separator.
 
-A tag name may contain lowercase and uppercase characters, digits, underscores,
+A tag name must be valid ASCII and may contain lowercase and uppercase letters,
-periods and dashes. A tag name may not start with a period or a dash and may
+digits, underscores, periods and dashes. A tag name may not start with a
-contain a maximum of 128 characters.
+period or a dash and may contain a maximum of 128 characters.
 
 You can group your images together using names and tags, and then upload them
 to [*Share Images via Repositories*](https://docs.docker.com/engine/tutorials/dockerrepos/#/contributing-to-docker-hub).

man/src/image/tag.md

@@ -9,15 +9,16 @@ entire image name including the optional `TAG` after the ':'.
    present, it may optionally be followed by a port number in the format 
    `:8080`. If not present, the command uses Docker's public registry located at
    `registry-1.docker.io` by default. Name components may contain lowercase 
-   characters, digits and separators. A separator is defined as a period, one or 
+   letters, digits and separators. A separator is defined as a period, one or
    two underscores, or one or more dashes. A name component may not start or end 
    with a separator.
 
 **TAG**
    The tag assigned to the image to version and distinguish images with the same
-   name. The tag name may contain lowercase and uppercase characters, digits, 
+   name. The tag name must be valid ASCII and may contain lowercase and
-   underscores, periods and dashes. A tag name may not start with a period or a 
+   uppercase letters, digits, underscores, periods and hyphens. A tag name
-   dash and may contain a maximum of 128 characters.
+   may not start with a period or a hyphen and may contain a maximum of 128
+   characters.
 
 # EXAMPLES