help: explain in `hg help flags` that unambiguous prefixes are allowed

I used `hg commit --amend` as an example because that's the most
frequently used flag I could think of that doesn't yet have a short
form.

Differential Revision: https://phab.mercurial-scm.org/D8607

mercurial/helptext/flags.txt

@@ -1,104 +1,106 b''
 Most Mercurial commands accept various flags.
 
 Flag names
 ==========
 
 Flags for each command are listed in :hg:`help` for that command.
 Additionally, some flags, such as --repository, are global and can be used with
 any command - those are seen in :hg:`help -v`, and can be specified before or
 after the command.
 
 Every flag has at least a long name, such as --repository. Some flags may also
 have a short one-letter name, such as the equivalent -R. Using the short or long
-name is equivalent and has the same effect.
+name is equivalent and has the same effect. The long name may be abbreviated to
+any unambiguous prefix. For example, :hg:`commit --amend` can be abbreviated
+to :hg:`commit --am`.
 
 Flags that have a short name can also be bundled together - for instance, to
 specify both --edit (short -e) and --interactive (short -i), one could use::
 
     hg commit -ei
 
 If any of the bundled flags takes a value (i.e. is not a boolean), it must be
 last, followed by the value::
 
     hg commit -im 'Message'
 
 Flag types
 ==========
 
 Mercurial command-line flags can be strings, numbers, booleans, or lists of
 strings.
 
 Specifying flag values
 ======================
 
 The following syntaxes are allowed, assuming a flag 'flagname' with short name
 'f'::
 
     --flagname=foo
     --flagname foo
     -f foo
     -ffoo
 
 This syntax applies to all non-boolean flags (strings, numbers or lists).
 
 Specifying boolean flags
 ========================
 
 Boolean flags do not take a value parameter. To specify a boolean, use the flag
 name to set it to true, or the same name prefixed with 'no-' to set it to
 false::
 
     hg commit --interactive
     hg commit --no-interactive
 
 Specifying list flags
 =====================
 
 List flags take multiple values. To specify them, pass the flag multiple times::
 
     hg files --include mercurial --include tests
 
 Setting flag defaults
 =====================
 
 In order to set a default value for a flag in an hgrc file, it is recommended to
 use aliases::
 
     [alias]
     commit = commit --interactive
 
 For more information on hgrc files, see :hg:`help config`.
 
 Overriding flags on the command line
 ====================================
 
 If the same non-list flag is specified multiple times on the command line, the
 latest specification is used::
 
     hg commit -m "Ignored value" -m "Used value"
 
 This includes the use of aliases - e.g., if one has::
 
     [alias]
     committemp = commit -m "Ignored value"
 
 then the following command will override that -m::
 
     hg committemp -m "Used value"
 
 Overriding flag defaults
 ========================
 
 Every flag has a default value, and you may also set your own defaults in hgrc
 as described above.
 Except for list flags, defaults can be overridden on the command line simply by
 specifying the flag in that location.
 
 Hidden flags
 ============
 
 Some flags are not shown in a command's help by default - specifically, those
 that are deemed to be experimental, deprecated or advanced. To show all flags,
 add the --verbose flag for the help command::
 
     hg help --verbose commit